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P0137 Code Explained: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) Causes, Symptoms & Fixes

P0137 code illustration showing a downstream oxygen sensor on Bank 1 Sensor 2 with a low-voltage signal, catalytic converter, Check Engine Light, Pro Street TV logo, and www.ProStreetOnline.com.

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:

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:

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:

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:

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:

Bank 1 Sensor 2

Located after the catalytic converter.

Used primarily for:

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:

The difference produces a voltage signal.

Typical operating range:

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:

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:

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:

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:

However, most P0137 cases involve:

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:

before replacing the sensor.

P0137 Quick Facts

ItemInformation
DTCP0137
Generic DefinitionO2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
SystemFuel & Emissions
Sensor LocationDownstream (After Catalytic Converter)
SeverityLow to Moderate
Safe to Drive?Usually Yes
Common SymptomsCheck Engine Light, emissions failure, catalyst monitor issues
Common CausesFailed downstream oxygen sensor, exhaust leaks, damaged wiring, connector corrosion
Typical Repair Cost$100–800+
Related CodesP0136, 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:

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:

Instead, it means the PCM has detected that Bank 1 Sensor 2 is producing a voltage signal lower than expected.

The cause may involve:

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:

Possible causes include:

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:

If voltage never rises, inspect:

Pattern 3: Sensor Drops to Zero Intermittently

Some vehicles show:

Possible causes include:

Intermittent faults often become worse as exhaust temperatures increase.

Pattern 4: Downstream Sensor Is Flat While Upstream Operates Normally

Monitor:

Normal results:

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:

Possible causes include:

Pattern 6: Catalyst Monitor Never Completes

Many scan tools show:

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:

Some drivers notice nothing except the illuminated warning light.

Check Engine Light

The Check Engine Light is the most common symptom.

Most manufacturers require:

before storing P0137.

Failed Emissions Inspection

P0137 frequently causes emissions failures because:

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:

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:

Fuel economy changes are generally less severe than with upstream oxygen sensor faults.

Slight Hesitation

Some vehicles may develop:

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:

Prompt repairs help maintain emissions compliance and proper catalyst monitoring.

Can You Drive With P0137?

Usually yes.

Driving is generally safe if:

Repairs should be scheduled promptly if:

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:

  1. Failed Bank 1 Sensor 2 oxygen sensor
  2. Exhaust leak before Sensor 2
  3. Open signal wire
  4. Signal wire shorted to ground
  5. Corroded connector
  6. Loose connector terminals
  7. Heat-damaged wiring
  8. Failed sensor heater
  9. Blown heater fuse
  10. Poor sensor ground
  11. Damaged wiring harness
  12. Water intrusion into connector
  13. Road debris damage
  14. Catalytic converter damage
  15. Restricted catalytic converter
  16. Lean engine operation
  17. Vacuum leak
  18. Low fuel pressure
  19. Dirty fuel injectors
  20. Contaminated oxygen sensor
  21. Oil contamination
  22. Coolant contamination
  23. Silicone contamination
  24. Improper aftermarket sensor
  25. Previous poor wiring repairs
  26. High circuit resistance
  27. PCM calibration issue
  28. Shared ground fault
  29. Exhaust modification affecting sensor readings
  30. 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 next section of this guide covers:

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:

Replacing the sensor before confirming the source of the low-voltage condition can lead to unnecessary repairs.

Safety Precautions

Before beginning diagnosis:

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:

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:

Monitor:

Do not erase the codes until all freeze-frame information has been saved.

Step 2 – Check for Related Trouble Codes

P0137 commonly appears with:

Multiple DTCs often help narrow the diagnosis.

Code CombinationLikely Diagnosis
P0137 onlyDownstream sensor or circuit fault
P0137 + P0141Heater circuit problem
P0137 + P0420Catalytic converter efficiency issue
P0137 + P0171Lean engine condition
P0137 + P0136General downstream sensor circuit problem

Step 3 – Verify Closed-Loop Operation

Allow the engine to reach operating temperature.

Monitor:

The PCM should transition from:

Open Loop

to

Closed Loop

If the PCM never enters closed loop, inspect:

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:

Watch for:

A consistently low signal is the defining characteristic of P0137.

Step 5 – Compare Upstream and Downstream Sensors

Monitor:

Under normal conditions:

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:

If voltage remains low:

Inspect:

Failure to respond usually indicates an electrical or sensor fault.

Step 7 – Analyze Fuel Trim Data

Monitor:

Large positive fuel trims may indicate:

Large negative trims may indicate:

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:

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:

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:

A short to ground is one of the most common electrical causes of P0137.

Step 11 – Verify Sensor Ground

Perform voltage-drop testing.

Inspect:

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:

A failed heater may delay sensor operation and contribute to persistent low-voltage readings.

Step 13 – Inspect the Exhaust System

Inspect the exhaust between:

Check for:

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:

If both sensors behave similarly, inspect the catalytic converter for reduced efficiency.

Step 15 – Inspect the Wiring Harness

Inspect the complete harness between:

Look for:

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:

Software updates occasionally resolve recurring diagnostic issues.

Step 17 – Compare Bank 2 Sensor 2 (If Equipped)

On V-engine applications:

Compare:

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:

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:

Freeze-frame data often reveals exactly when the low-voltage condition occurred.

Step 20 – Confirm the Diagnosis

Before replacing components, verify:

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:

  1. Allow the exhaust system to cool completely.
  2. Disconnect the negative battery cable if recommended by the manufacturer.
  3. Disconnect the Bank 1 Sensor 2 electrical connector.
  4. Remove the sensor using an oxygen sensor socket.
  5. Inspect the mounting threads and sensor bung.
  6. Install a quality OEM or OEM-equivalent replacement sensor.
  7. Tighten the sensor to the manufacturer’s specified torque.
  8. Reconnect the electrical connector.
  9. Clear all stored diagnostic trouble codes.
  10. 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:

Repair wiring using:

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:

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:

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:

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:

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:

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:

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:

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:

PCM replacement should always be the final diagnostic step.

How to Fix P0137

The proper repair depends entirely on diagnostic results.

Common repairs include:

Verifying the Repair

After completing repairs:

  1. Clear all diagnostic trouble codes.
  2. Start the engine.
  3. Allow the engine to reach operating temperature.
  4. Verify closed-loop operation.
  5. Monitor Bank 1 Sensor 2 voltage.
  6. Confirm the voltage no longer remains below approximately 0.2 volts.
  7. Compare upstream and downstream sensor activity.
  8. Complete a full road test.
  9. Verify catalyst and oxygen sensor readiness monitors complete successfully.
  10. Confirm no pending or permanent DTCs remain.

A successful repair restores normal downstream oxygen sensor voltage and proper catalyst monitoring.

Typical Repair Costs

RepairTypical 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:

Many P0137 repairs involve correcting wiring, exhaust, or engine issues rather than replacing the oxygen sensor itself.

Manufacturer-Specific Notes

General Motors

Inspect:

Heat-damaged wiring is common on older GM trucks and SUVs.

Ford

Verify:

EcoBoost engines should also be inspected for turbocharger-related exhaust leaks.

Toyota / Lexus

Inspect:

Toyota vehicles often respond best to OEM oxygen sensors.

Honda / Acura

Inspect:

Aftermarket sensors occasionally produce inaccurate voltage characteristics.

Nissan / Infiniti

Compare:

Corrosion and road debris commonly damage downstream sensor wiring.

Hyundai / Kia

Inspect:

Road salt frequently contributes to connector corrosion.

Volkswagen / Audi

Verify:

Turbocharged engines require careful inspection of post-catalyst exhaust components.

BMW / MINI

Use factory diagnostic software to monitor:

Many BMW applications require manufacturer-specific diagnostic routines.

Chrysler / Dodge / Jeep / Ram

Inspect:

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:

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.

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