If your OBD-II scanner displays P0137, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected that the Bank 1 Sensor 2 oxygen (O2) sensor is producing a voltage signal that is lower than the manufacturer’s acceptable range.
The generic OBD-II definition for P0137 is:
O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
Bank 1 Sensor 2 is the downstream oxygen sensor, located after the catalytic converter on the engine bank containing cylinder number one.
Unlike Bank 1 Sensor 1, which directly controls the air-fuel ratio, Bank 1 Sensor 2 primarily monitors:
- Catalytic converter efficiency
- Oxygen storage capacity
- Exhaust emissions
- OBD-II catalyst readiness
- Overall converter performance
When the PCM detects that the downstream oxygen sensor remains at an abnormally low voltage for longer than expected, it stores P0137 and usually illuminates the Check Engine Light.
Although a failed oxygen sensor is a common cause, low voltage can also result from:
- Exhaust leaks
- Damaged wiring
- Poor electrical connections
- Sensor contamination
- Heater circuit faults
- Ground problems
- Catalytic converter issues
Replacing the sensor before testing the circuit is often the fastest way to discover the problem was somewhere else.
What Does the P0137 Code Mean?
The downstream oxygen sensor continuously measures the amount of oxygen remaining in the exhaust gases after they pass through the catalytic converter.
Unlike the upstream sensor, which rapidly switches between rich and lean conditions, the downstream sensor normally produces a more stable voltage signal because the catalytic converter smooths out oxygen fluctuations.
With P0137, the PCM has determined that the downstream sensor voltage remains too low for an extended period.
Typically, the voltage may remain below approximately:
0.2 volts
instead of varying normally.
A consistently low voltage suggests the PCM believes one of the following has occurred:
- The sensor is reporting a lean exhaust condition continuously.
- The signal circuit has an electrical fault.
- The sensor cannot generate a proper voltage.
- The wiring is damaged.
- The connector has poor electrical contact.
P0137 identifies a low-voltage condition, not necessarily a failed oxygen sensor.
Where Is Bank 1 Sensor 2 Located?
Bank 1 Sensor 2 is always located after the catalytic converter.
Depending on vehicle design, it may be installed in the:
- Catalytic converter outlet
- Intermediate exhaust pipe
- Front exhaust section
- Exhaust tunnel beneath the vehicle
On inline engines:
Bank 1 is the only engine bank.
On V-engines:
Bank 1 is the side containing cylinder number one.
Sensor 2 always refers to the downstream oxygen sensor located behind the catalytic converter.
Bank 1 Sensor 1 vs. Bank 1 Sensor 2
Understanding sensor locations prevents unnecessary parts replacement.
Bank 1 Sensor 1
Located before the catalytic converter.
Used primarily for:
- Fuel control
- Air-fuel ratio adjustments
- Short-Term Fuel Trim
- Long-Term Fuel Trim
- Closed-loop fuel operation
Bank 1 Sensor 2
Located after the catalytic converter.
Used primarily for:
- Catalyst efficiency monitoring
- Oxygen storage evaluation
- Emissions verification
- OBD-II readiness monitoring
Confusing these two sensors is one of the most common oxygen sensor repair mistakes.
How the Downstream Oxygen Sensor Works
Most downstream oxygen sensors are zirconia narrowband sensors.
The sensing element compares:
- Oxygen in the exhaust gases
- Oxygen in outside air
The difference produces a voltage signal.
Typical operating range:
- Lean: approximately 0.1–0.2 volts
- Rich: approximately 0.8–0.9 volts
Because the catalytic converter smooths exhaust oxygen fluctuations, the downstream sensor normally changes voltage much more gradually than the upstream sensor.
What Does “Low Voltage” Mean?
A low-voltage signal indicates the PCM is receiving less voltage than expected from Bank 1 Sensor 2.
Possible causes include:
- Actual lean exhaust
- Exhaust leak
- Failed oxygen sensor
- Open signal circuit
- Short to ground
- Poor ground connection
- Connector corrosion
- Sensor contamination
The PCM cannot determine which condition exists—it only detects that the voltage remains below the allowable threshold.
How the PCM Detects P0137
The PCM monitors the downstream oxygen sensor under specific operating conditions.
Typical monitoring criteria include:
- Closed-loop operation
- Engine coolant temperature
- Engine load
- Vehicle speed
- Oxygen sensor heater status
- Sensor voltage
- Catalytic converter performance
If Bank 1 Sensor 2 remains below the calibrated voltage threshold for too long, the PCM stores P0137.
Some manufacturers require multiple failed drive cycles before illuminating the Check Engine Light.
Why the Downstream Oxygen Sensor Matters
Although Bank 1 Sensor 2 does not directly control injector pulse width like the upstream sensor, it performs several critical functions.
It helps the PCM verify:
- Catalytic converter efficiency
- Emissions-system performance
- Oxygen storage capability
- Catalyst monitor completion
- Overall emissions compliance
Without reliable downstream oxygen sensor data, the PCM cannot accurately evaluate catalytic converter performance.
Can a Lean Engine Cause P0137?
Yes—but not always.
An actual lean-running engine can lower downstream oxygen sensor voltage.
Possible causes include:
- Vacuum leaks
- Low fuel pressure
- Restricted fuel injectors
- Intake leaks
However, most P0137 cases involve:
- Sensor faults
- Wiring damage
- Exhaust leaks
- Electrical problems
Proper diagnosis should determine whether the low voltage is caused by the engine or the sensor circuit.
Can an Exhaust Leak Cause P0137?
Absolutely.
An exhaust leak before or near Bank 1 Sensor 2 allows fresh outside air into the exhaust stream.
This extra oxygen can cause the downstream sensor to report an artificially lean condition.
Inspect:
- Exhaust flanges
- Welds
- Pipe connections
- Catalytic converter joints
before replacing the sensor.
P0137 Quick Facts
| Item | Information |
|---|---|
| DTC | P0137 |
| Generic Definition | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) |
| System | Fuel & Emissions |
| Sensor Location | Downstream (After Catalytic Converter) |
| Severity | Low to Moderate |
| Safe to Drive? | Usually Yes |
| Common Symptoms | Check Engine Light, emissions failure, catalyst monitor issues |
| Common Causes | Failed downstream oxygen sensor, exhaust leaks, damaged wiring, connector corrosion |
| Typical Repair Cost | $100–800+ |
| Related Codes | P0136, P0138, P0139, P0140, P0141, P0420 |
Common Vehicles That Experience P0137
Because P0137 is a generic OBD-II diagnostic trouble code, it may occur on nearly every gasoline-powered vehicle manufactured since 1996.
Common manufacturers include:
- Chevrolet
- GMC
- Buick
- Cadillac
- Ford
- Lincoln
- Toyota
- Lexus
- Honda
- Acura
- Nissan
- Infiniti
- Hyundai
- Kia
- Mazda
- Subaru
- Volkswagen
- Audi
- BMW
- MINI
- Mercedes-Benz
- Chrysler
- Dodge
- Jeep
- Ram
Vehicles driven in wet or snowy climates often experience connector corrosion or wiring damage that contributes to P0137.
What Does P0137 Really Mean?
The wording of P0137 often leads people to assume the oxygen sensor has failed.
The code does not automatically mean:
- The oxygen sensor is defective.
- The catalytic converter is bad.
- The engine is permanently lean.
- The PCM has failed.
Instead, it means the PCM has detected that Bank 1 Sensor 2 is producing a voltage signal lower than expected.
The cause may involve:
- The oxygen sensor
- Wiring
- Connectors
- Ground circuits
- Exhaust leaks
- Heater operation
- Actual lean exhaust conditions
Think of P0137 as the PCM saying:
“The downstream oxygen sensor is telling me the voltage is too low—or something is preventing me from seeing a proper signal.”
Common P0137 Scan Tool Patterns
Monitoring live scan-tool data is one of the quickest ways to diagnose P0137.
Unlike the upstream oxygen sensor, Bank 1 Sensor 2 should produce a relatively stable voltage because it monitors exhaust gases after they pass through the catalytic converter.
A properly functioning downstream sensor usually changes voltage slowly and remains significantly more stable than Bank 1 Sensor 1.
P0137 occurs when the PCM detects that the downstream oxygen sensor voltage remains abnormally low for longer than the manufacturer’s allowable limit.
Pattern 1: Downstream Sensor Stays Below 0.2 Volts
The most common scan-tool pattern is a downstream oxygen sensor that remains near:
- 0.00 volts
- 0.05 volts
- 0.10 volts
- Below approximately 0.20 volts
Possible causes include:
- Failed oxygen sensor
- Exhaust leak
- Open signal circuit
- Short to ground
- Poor connector connection
This is the classic live-data pattern associated with P0137.
Pattern 2: Sensor Never Responds to Rich Conditions
Momentarily enriching the air-fuel mixture should cause Bank 1 Sensor 2 voltage to increase.
A healthy sensor should:
- Rise toward 0.8–0.9 volts
- Respond gradually
- Stabilize after enrichment
If voltage never rises, inspect:
- Sensor operation
- Signal wiring
- Ground circuit
- Connector integrity
Pattern 3: Sensor Drops to Zero Intermittently
Some vehicles show:
- Normal voltage
- Sudden drop to zero
- Immediate recovery
- Repeated signal interruptions
Possible causes include:
- Loose connector
- Broken wire
- Heat-damaged harness
- Intermittent ground
Intermittent faults often become worse as exhaust temperatures increase.
Pattern 4: Downstream Sensor Is Flat While Upstream Operates Normally
Monitor:
- Bank 1 Sensor 1
- Bank 1 Sensor 2
Normal results:
- Sensor 1 switches rapidly.
- Sensor 2 remains stable but active.
If Sensor 1 operates correctly while Sensor 2 remains fixed at very low voltage, the fault is usually isolated to the downstream sensor or its circuit.
Pattern 5: Low Voltage Appears Only After Warm-Up
The oxygen sensor may appear normal immediately after startup.
Once closed-loop operation begins:
- Voltage gradually falls
- Signal becomes weak
- P0137 sets
Possible causes include:
- Weak heater circuit
- Aging sensor
- High-resistance wiring
- Internal sensor failure
Pattern 6: Catalyst Monitor Never Completes
Many scan tools show:
- Catalyst Monitor: Incomplete
- Oxygen Sensor Monitor: Failed
Because the PCM cannot trust Bank 1 Sensor 2, catalyst monitoring may stop until repairs are completed.
Common Symptoms of P0137
Many vehicles experience only minor drivability symptoms because the downstream oxygen sensor does not directly control injector pulse width.
Common symptoms include:
- Check Engine Light
- Failed emissions inspection
- Catalyst monitor incomplete
- Increased emissions
- Slight reduction in fuel economy
- Occasional hesitation
- Additional oxygen sensor codes
- No noticeable drivability problems
Some drivers notice nothing except the illuminated warning light.
Check Engine Light
The Check Engine Light is the most common symptom.
Most manufacturers require:
- Multiple failed drive cycles
- Closed-loop operation
- Confirmed low-voltage readings
before storing P0137.
Failed Emissions Inspection
P0137 frequently causes emissions failures because:
- The MIL is illuminated.
- Readiness monitors remain incomplete.
- Catalyst efficiency cannot be verified.
Even if the engine runs perfectly, emissions testing usually disagrees.
Catalyst Monitor Problems
The downstream oxygen sensor verifies catalytic converter performance.
When its voltage remains too low:
- Catalyst monitor may not run.
- Catalyst monitor may fail.
- Readiness monitors remain incomplete.
This often delays registration renewal in emissions-controlled states.
Reduced Fuel Economy
Although the downstream oxygen sensor is not primarily used for fuel control, some vehicles use its information for adaptive fuel strategies.
Drivers may notice:
- Slightly lower MPG
- Increased fuel consumption
- Minor reductions in efficiency
Fuel economy changes are generally less severe than with upstream oxygen sensor faults.
Slight Hesitation
Some vehicles may develop:
- Mild hesitation
- Slight throttle delay
- Occasional rough idle
These symptoms often indicate another engine problem contributing to P0137 rather than the downstream sensor alone.
Is P0137 Serious?
P0137 is generally considered a low-to-moderate severity diagnostic trouble code.
The vehicle usually remains fully drivable.
However, ignoring the code may lead to:
- Increased emissions
- Failed emissions testing
- Incomplete readiness monitors
- Undetected catalytic converter problems
- Additional oxygen sensor codes
Prompt repairs help maintain emissions compliance and proper catalyst monitoring.
Can You Drive With P0137?
Usually yes.
Driving is generally safe if:
- The engine runs normally.
- No flashing Check Engine Light is present.
- No severe misfire exists.
- Vehicle performance remains acceptable.
Repairs should be scheduled promptly if:
- P0420 also appears.
- Fuel economy decreases significantly.
- Additional oxygen sensor codes develop.
- Exhaust leaks become noticeable.
P0137 vs Related Trouble Codes
Understanding similar downstream oxygen sensor codes simplifies diagnosis.
P0136
O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
General circuit fault.
P0137
O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
Voltage remains too low.
P0138
O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
Voltage remains too high.
P0139
O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
Sensor responds too slowly.
P0140
O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
No usable sensor signal.
P0141
O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
Heater circuit failure.
P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Catalytic converter performance problem.
30 Common Causes of P0137
The most common causes include:
- Failed Bank 1 Sensor 2 oxygen sensor
- Exhaust leak before Sensor 2
- Open signal wire
- Signal wire shorted to ground
- Corroded connector
- Loose connector terminals
- Heat-damaged wiring
- Failed sensor heater
- Blown heater fuse
- Poor sensor ground
- Damaged wiring harness
- Water intrusion into connector
- Road debris damage
- Catalytic converter damage
- Restricted catalytic converter
- Lean engine operation
- Vacuum leak
- Low fuel pressure
- Dirty fuel injectors
- Contaminated oxygen sensor
- Oil contamination
- Coolant contamination
- Silicone contamination
- Improper aftermarket sensor
- Previous poor wiring repairs
- High circuit resistance
- PCM calibration issue
- Shared ground fault
- Exhaust modification affecting sensor readings
- PCM failure (rare)
Final Thoughts
The P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) code indicates the PCM has detected that the downstream oxygen sensor voltage has remained below its expected operating range.
While a faulty oxygen sensor is a common cause, exhaust leaks, damaged wiring, poor grounds, heater circuit failures, connector corrosion, lean operating conditions, and catalytic converter issues can all produce the same low-voltage condition.
Accurate diagnosis should determine whether the fault originates in:
- The oxygen sensor
- The wiring harness
- The connector
- The exhaust system
- The catalytic converter
- The engine itself
The next section of this guide covers:
- Complete diagnostic procedures
- Live-data analysis
- Oxygen sensor voltage testing
- Heater circuit testing
- Exhaust leak diagnosis
- Catalytic converter evaluation
- Repair procedures
- Repair costs
- Manufacturer-specific troubleshooting
- Final repair verification
Following a systematic diagnostic process prevents unnecessary oxygen sensor and catalytic converter replacement while restoring accurate emissions monitoring and OBD-II readiness.
How to Diagnose the P0137 Code
Diagnosing P0137 requires determining why the Bank 1 Sensor 2 oxygen sensor voltage remains below the acceptable operating range.
Unlike the upstream oxygen sensor, which directly controls fuel delivery, the downstream oxygen sensor primarily verifies catalytic converter efficiency and helps the PCM monitor emissions performance.
Although a failed downstream oxygen sensor is one of the most common causes of P0137, low voltage may also result from:
- Exhaust leaks
- Damaged wiring
- Poor grounds
- Connector corrosion
- Heater circuit failures
- Lean engine operation
- Catalytic converter problems
Replacing the sensor before confirming the source of the low-voltage condition can lead to unnecessary repairs.
Safety Precautions
Before beginning diagnosis:
- Park the vehicle on level ground.
- Set the parking brake.
- Allow the exhaust system to cool completely.
- Wear safety glasses.
- Use insulated gloves when working near exhaust components.
- Disconnect the battery before repairing electrical wiring.
The downstream oxygen sensor operates in an extremely hot environment and can exceed 900°F (480°C) during normal operation.
Tools Required
Professional diagnosis may require:
- Professional scan tool
- Live-data capability
- Digital multimeter
- Oscilloscope (recommended)
- Oxygen sensor socket
- Smoke machine
- Fuel pressure gauge
- Infrared thermometer
- Back-probe leads
- Wiring diagrams
- Factory service information
- Basic hand tools
A graphing scan tool or oscilloscope provides the clearest view of downstream oxygen sensor voltage behavior.
Step 1 – Verify the Code
Connect the scan tool.
Record:
- Stored DTCs
- Pending DTCs
- Permanent DTCs
- Freeze-frame data
Monitor:
- Bank 1 Sensor 2 voltage
- Bank 1 Sensor 1 voltage
- STFT
- LTFT
- Engine Coolant Temperature
- Engine Load
- Fuel System Status
Do not erase the codes until all freeze-frame information has been saved.
Step 2 – Check for Related Trouble Codes
P0137 commonly appears with:
- P0136
- P0138
- P0139
- P0140
- P0141
- P0171
- P0172
- P0420
Multiple DTCs often help narrow the diagnosis.
| Code Combination | Likely Diagnosis |
|---|---|
| P0137 only | Downstream sensor or circuit fault |
| P0137 + P0141 | Heater circuit problem |
| P0137 + P0420 | Catalytic converter efficiency issue |
| P0137 + P0171 | Lean engine condition |
| P0137 + P0136 | General downstream sensor circuit problem |
Step 3 – Verify Closed-Loop Operation
Allow the engine to reach operating temperature.
Monitor:
- Fuel System Status
The PCM should transition from:
Open Loop
to
Closed Loop
If the PCM never enters closed loop, inspect:
- Upstream oxygen sensor
- Engine Coolant Temperature sensor
- Thermostat
- PCM calibration
Downstream oxygen sensor testing should always be performed during closed-loop operation.
Step 4 – Monitor Bank 1 Sensor 2 Voltage
Observe live data.
A healthy downstream oxygen sensor should:
- Produce a relatively stable signal
- Respond gradually to changing exhaust conditions
- Operate significantly differently than the upstream sensor
Watch for:
- Constant low voltage
- Voltage below approximately 0.2 volts
- Little or no activity
- Intermittent dropouts
A consistently low signal is the defining characteristic of P0137.
Step 5 – Compare Upstream and Downstream Sensors
Monitor:
- Bank 1 Sensor 1
- Bank 1 Sensor 2
Under normal conditions:
- Sensor 1 switches rapidly.
- Sensor 2 changes slowly.
If Sensor 2 remains extremely low while Sensor 1 operates normally, inspect the downstream sensor and its circuit.
If both sensors remain lean, investigate possible engine or fuel delivery problems.
Step 6 – Perform a Rich Condition Test
Momentarily enrich the air-fuel mixture.
A healthy downstream sensor should gradually increase toward:
- 0.8–0.9 volts
If voltage remains low:
Inspect:
- Oxygen sensor
- Signal circuit
- Connector
- Ground
Failure to respond usually indicates an electrical or sensor fault.
Step 7 – Analyze Fuel Trim Data
Monitor:
- Short-Term Fuel Trim (STFT)
- Long-Term Fuel Trim (LTFT)
Large positive fuel trims may indicate:
- Vacuum leaks
- Intake leaks
- Low fuel pressure
Large negative trims may indicate:
- Rich operation
- Fuel injector leakage
- Excessive fuel pressure
Fuel trim data helps determine whether the low voltage reflects an actual lean condition or a sensor circuit problem.
Step 8 – Inspect the Oxygen Sensor
Inspect Bank 1 Sensor 2 for:
- Carbon buildup
- Oil contamination
- Coolant contamination
- Silicone contamination
- Physical damage
- Impact damage
Road debris commonly damages downstream oxygen sensors because of their location beneath the vehicle.
Step 9 – Inspect the Electrical Connector
Disconnect the connector.
Inspect for:
- Corrosion
- Loose terminals
- Bent pins
- Water intrusion
- Broken locking tabs
- Heat damage
Poor electrical contact frequently causes low-voltage conditions.
Step 10 – Test the Signal Circuit
Using a digital multimeter:
Back-probe the signal wire.
Inspect for:
- Open circuits
- High resistance
- Short to ground
- Short to voltage
- Intermittent voltage loss
A short to ground is one of the most common electrical causes of P0137.
Step 11 – Verify Sensor Ground
Perform voltage-drop testing.
Inspect:
- Sensor ground
- Chassis ground
- Engine ground
- PCM ground
Poor ground quality can significantly reduce oxygen sensor output voltage.
Step 12 – Test the Heater Circuit
The downstream oxygen sensor contains an internal heater.
Inspect:
- Heater power
- Heater ground
- Heater resistance
- Heater fuse
- Heater relay
A failed heater may delay sensor operation and contribute to persistent low-voltage readings.
Step 13 – Inspect the Exhaust System
Inspect the exhaust between:
- Catalytic converter
- Bank 1 Sensor 2
Check for:
- Exhaust leaks
- Cracked pipes
- Loose flanges
- Damaged gaskets
- Weld failures
Fresh air entering the exhaust stream can falsely lower downstream oxygen sensor voltage.
Step 14 – Evaluate Catalytic Converter Performance
Compare upstream and downstream oxygen sensor activity.
A healthy catalytic converter should:
- Smooth oxygen fluctuations
- Produce a stable downstream signal
- Reduce downstream switching
If both sensors behave similarly, inspect the catalytic converter for reduced efficiency.
Step 15 – Inspect the Wiring Harness
Inspect the complete harness between:
- Bank 1 Sensor 2
- PCM
Look for:
- Melted insulation
- Chafed wiring
- Rodent damage
- Previous repairs
- Contact with exhaust components
Heat damage is one of the leading causes of downstream oxygen sensor circuit faults.
Step 16 – Review Technical Service Bulletins
Before replacing expensive components:
Review manufacturer Technical Service Bulletins.
Look for updates involving:
- Oxygen sensor diagnostics
- Catalyst monitoring
- Heater logic
- PCM software
- False P0137 detection
Software updates occasionally resolve recurring diagnostic issues.
Step 17 – Compare Bank 2 Sensor 2 (If Equipped)
On V-engine applications:
Compare:
- Bank 1 Sensor 2
- Bank 2 Sensor 2
If Bank 2 operates normally while Bank 1 remains at low voltage, the fault is likely isolated to Bank 1.
Step 18 – Inspect the Catalytic Converter
Inspect for:
- Physical damage
- Restriction
- Overheating
- Internal rattling
- External discoloration
Although P0137 is primarily a sensor circuit code, converter damage can influence downstream oxygen sensor readings.
Step 19 – Review Freeze-Frame Data
Compare current live data with stored freeze-frame information.
Review:
- Engine RPM
- Engine Load
- Coolant Temperature
- Vehicle Speed
- Oxygen sensor voltage
- Fuel trims
Freeze-frame data often reveals exactly when the low-voltage condition occurred.
Step 20 – Confirm the Diagnosis
Before replacing components, verify:
- Oxygen sensor voltage
- Signal circuit continuity
- Heater operation
- Ground integrity
- Connector condition
- Exhaust integrity
- Fuel trim data
- Catalytic converter performance
A complete diagnosis prevents unnecessary replacement of oxygen sensors and catalytic converters.
Step 21 – Replace the Downstream Oxygen Sensor (If Required)
Replace Bank 1 Sensor 2 only after confirming that the downstream oxygen sensor is unable to produce the correct voltage or that testing has verified an internal failure.
General replacement procedure:
- Allow the exhaust system to cool completely.
- Disconnect the negative battery cable if recommended by the manufacturer.
- Disconnect the Bank 1 Sensor 2 electrical connector.
- Remove the sensor using an oxygen sensor socket.
- Inspect the mounting threads and sensor bung.
- Install a quality OEM or OEM-equivalent replacement sensor.
- Tighten the sensor to the manufacturer’s specified torque.
- Reconnect the electrical connector.
- Clear all stored diagnostic trouble codes.
- Verify proper sensor operation with a scan tool.
OEM sensors typically provide the most accurate voltage characteristics and catalyst-monitor performance.
Step 22 – Repair the Signal Circuit
A damaged signal wire is one of the leading causes of P0137.
Inspect the circuit for:
- Open wiring
- Short to ground
- High resistance
- Melted insulation
- Chafed harnesses
- Rodent damage
- Previous poor-quality repairs
Repair wiring using:
- Automotive-grade wire
- Weatherproof connectors
- Adhesive-lined heat shrink
- Proper harness routing away from exhaust heat
A signal wire grounded against the exhaust system commonly produces a constant low-voltage condition.
Step 23 – Repair Connector Problems
Disconnect the downstream oxygen sensor connector.
Inspect for:
- Green corrosion
- Moisture intrusion
- Bent terminals
- Loose pins
- Broken locking tabs
- Heat damage
A poor connector can reduce signal voltage even when the oxygen sensor itself is functioning properly.
Replace damaged connectors rather than attempting temporary repairs.
Step 24 – Repair the Heater Circuit
Although P0141 is the primary heater-circuit code, heater failures may contribute to P0137.
Inspect:
- Heater power supply
- Heater ground
- Heater resistance
- Oxygen sensor fuse
- Heater relay
If the heater never warms the sensor properly, voltage output may remain lower than expected.
Repair the heater circuit before replacing additional components.
Step 25 – Repair Exhaust Leaks
Inspect the exhaust system around Bank 1 Sensor 2.
Repair:
- Cracked exhaust pipes
- Damaged catalytic converter flanges
- Loose clamps
- Leaking gaskets
- Cracked welds
Fresh outside air entering the exhaust stream lowers downstream oxygen sensor voltage and may falsely indicate a lean exhaust condition.
Step 26 – Repair Lean Engine Conditions
If fuel trim analysis confirms the engine is actually running lean, repair the underlying cause before replacing the oxygen sensor.
Inspect for:
- Vacuum leaks
- Intake manifold leaks
- Low fuel pressure
- Restricted fuel injectors
- Weak fuel pump
- Dirty Mass Air Flow (MAF) sensor
Correcting the lean condition often restores normal downstream oxygen sensor voltage.
Step 27 – Repair Ground Circuit Problems
Poor grounds frequently reduce oxygen sensor voltage.
Perform voltage-drop testing.
Inspect:
- Engine grounds
- Chassis grounds
- PCM grounds
- Oxygen sensor ground circuit
Clean, tighten, or replace damaged ground connections as necessary.
Step 28 – Inspect the Catalytic Converter
P0137 is primarily a downstream sensor code, but converter problems can affect sensor voltage.
Inspect for:
- Physical damage
- Restriction
- Internal substrate failure
- Overheating
- External discoloration
If P0420 is also present, perform catalyst efficiency testing before replacing the converter.
Step 29 – Update PCM Software
Before replacing expensive components, review manufacturer Technical Service Bulletins (TSBs).
Some manufacturers have released PCM updates addressing:
- Oxygen sensor diagnostics
- Catalyst monitor calibration
- Heater circuit logic
- False P0137 detection
- Emissions monitor strategy
Always verify the PCM has the latest available software.
Step 30 – Consider PCM Failure
PCM failure is extremely uncommon.
Only suspect the PCM after confirming:
- Oxygen sensor operation
- Signal circuit integrity
- Heater operation
- Wiring condition
- Connector integrity
- Ground quality
- Exhaust system integrity
- Catalytic converter performance
- PCM software updates
PCM replacement should always be the final diagnostic step.
How to Fix P0137
The proper repair depends entirely on diagnostic results.
Common repairs include:
- Replacing Bank 1 Sensor 2 oxygen sensor
- Repairing damaged wiring
- Replacing corroded connectors
- Repairing the heater circuit
- Repairing exhaust leaks
- Repairing vacuum leaks
- Cleaning or replacing the MAF sensor
- Repairing fuel delivery problems
- Repairing ground circuits
- Updating PCM software
- Replacing the catalytic converter (only if testing confirms failure)
- Replacing the PCM (rare)
Verifying the Repair
After completing repairs:
- Clear all diagnostic trouble codes.
- Start the engine.
- Allow the engine to reach operating temperature.
- Verify closed-loop operation.
- Monitor Bank 1 Sensor 2 voltage.
- Confirm the voltage no longer remains below approximately 0.2 volts.
- Compare upstream and downstream sensor activity.
- Complete a full road test.
- Verify catalyst and oxygen sensor readiness monitors complete successfully.
- Confirm no pending or permanent DTCs remain.
A successful repair restores normal downstream oxygen sensor voltage and proper catalyst monitoring.
Typical Repair Costs
| Repair | Typical Cost |
|---|---|
| Professional diagnosis | $100–250 |
| Bank 1 Sensor 2 replacement | $175–450 |
| Wiring repair | $150–500 |
| Connector replacement | $100–350 |
| Heater circuit repair | $150–500 |
| Exhaust leak repair | $150–800 |
| Vacuum leak repair | $150–600 |
| Fuel system repair | $200–900 |
| PCM software update | $100–300 |
| Catalytic converter replacement | $900–2,800+ |
| PCM replacement | $1,000–3,000+ |
Repair costs vary based on vehicle make, labor rates, and component accessibility.
Common Diagnostic Mistakes
Avoid these common mistakes:
- Replacing the downstream oxygen sensor without testing the signal circuit
- Confusing Bank 1 Sensor 2 with Bank 1 Sensor 1
- Ignoring exhaust leaks
- Overlooking lean engine conditions
- Skipping heater circuit testing
- Failing to inspect ground circuits
- Ignoring freeze-frame data
- Replacing the catalytic converter without confirming failure
- Skipping PCM software updates
- Replacing the PCM before eliminating simpler faults
Many P0137 repairs involve correcting wiring, exhaust, or engine issues rather than replacing the oxygen sensor itself.
Manufacturer-Specific Notes
General Motors
Inspect:
- Downstream oxygen sensor voltage
- Heater circuit
- Wiring near the transmission tunnel
- Exhaust leaks behind the catalytic converter
Heat-damaged wiring is common on older GM trucks and SUVs.
Ford
Verify:
- Heater voltage
- Downstream sensor response
- Exhaust flange integrity
- Catalyst monitor completion
EcoBoost engines should also be inspected for turbocharger-related exhaust leaks.
Toyota / Lexus
Inspect:
- Downstream oxygen sensor
- Air-Fuel Ratio (A/F) sensor operation
- Heater circuit
- Connector condition
Toyota vehicles often respond best to OEM oxygen sensors.
Honda / Acura
Inspect:
- Downstream sensor voltage
- Heater circuit
- Exhaust pipe connections
- Ground circuits
Aftermarket sensors occasionally produce inaccurate voltage characteristics.
Nissan / Infiniti
Compare:
- Upstream and downstream sensor voltages
- Fuel trims
- MAF sensor readings
- Connector integrity
Corrosion and road debris commonly damage downstream sensor wiring.
Hyundai / Kia
Inspect:
- Heater fuse
- Connector condition
- Wiring beneath the vehicle
- Exhaust system integrity
Road salt frequently contributes to connector corrosion.
Volkswagen / Audi
Verify:
- Rear oxygen sensor operation
- Heater activation
- Catalyst monitor
- Exhaust integrity
Turbocharged engines require careful inspection of post-catalyst exhaust components.
BMW / MINI
Use factory diagnostic software to monitor:
- Lambda sensor voltage
- Catalyst efficiency
- Heater operation
- Readiness monitors
Many BMW applications require manufacturer-specific diagnostic routines.
Chrysler / Dodge / Jeep / Ram
Inspect:
- Downstream sensor wiring
- Heater circuit
- Exhaust system
- Connector quality
Exhaust heat often damages the rear oxygen sensor harness.
Frequently Asked Questions
What usually fixes P0137?
The most common repairs include replacing the downstream oxygen sensor, repairing damaged wiring, fixing exhaust leaks, repairing the heater circuit, restoring proper grounds, or correcting a lean-running engine.
Can I drive with P0137?
Yes. Most vehicles remain drivable, but emissions may increase, catalyst monitoring may stop, and the vehicle may fail an emissions inspection until repairs are completed.
Does P0137 always mean the oxygen sensor is bad?
No. Wiring damage, exhaust leaks, poor grounds, connector corrosion, heater failures, and actual lean engine conditions can all produce a low-voltage signal.
Can an exhaust leak cause P0137?
Yes. Fresh air entering the exhaust near Bank 1 Sensor 2 lowers oxygen sensor voltage and commonly triggers P0137.
Can a lean engine cause P0137?
Absolutely. Vacuum leaks, low fuel pressure, intake leaks, or restricted injectors may create a true lean exhaust condition that lowers downstream sensor voltage.
Can a bad catalytic converter cause P0137?
Sometimes, but it is not the most common cause. Catalytic converter failures typically trigger P0420, although severe converter damage can influence downstream oxygen sensor readings.
Final Thoughts
The P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) code indicates the PCM has detected that the downstream oxygen sensor voltage has remained below the expected operating range. While a failed oxygen sensor is a common cause, exhaust leaks, damaged wiring, poor grounds, heater circuit failures, lean engine conditions, and catalytic converter problems can all produce the same fault.
A complete diagnosis should always verify:
- Downstream oxygen sensor voltage
- Signal circuit continuity
- Heater operation
- Ground integrity
- Wiring condition
- Exhaust system integrity
- Fuel trim data
- Catalytic converter performance
- PCM software level
Following a structured diagnostic process restores accurate emissions monitoring, prevents unnecessary oxygen sensor and catalytic converter replacement, and ensures the vehicle’s OBD-II readiness monitors complete successfully.
