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P0138 Code Explained: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) Causes, Symptoms & Fixes

P0138 code illustration showing a downstream oxygen sensor on Bank 1 Sensor 2 with a high-voltage signal, catalytic converter, Check Engine Light, Pro Street TV logo, and www.ProStreetOnline.com.

If your OBD-II scanner displays P0138, 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 higher than the manufacturer’s acceptable operating range.

The generic OBD-II definition for P0138 is:

O2 Sensor Circuit High 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 unusually high voltage for longer than expected, it stores P0138 and usually illuminates the Check Engine Light.

Although a failed oxygen sensor is a common cause, high voltage may also result from:

Replacing the oxygen sensor before determining why the voltage is high is an excellent way to collect spare oxygen sensors you probably didn’t need.

What Does the P0138 Code Mean?

The downstream oxygen sensor measures the amount of oxygen remaining in the exhaust gases after they pass through the catalytic converter.

Unlike the upstream oxygen sensor, which rapidly switches between lean and rich conditions, the downstream sensor normally produces a more stable voltage signal because the catalytic converter smooths oxygen fluctuations.

With P0138, the PCM has determined that the downstream oxygen sensor voltage remains too high for an extended period.

Typically, the voltage remains near:

instead of changing gradually within its expected operating range.

A consistently high voltage suggests one of the following:

P0138 identifies a high-voltage condition, not automatically a failed oxygen sensor.

Where Is Bank 1 Sensor 2 Located?

Bank 1 Sensor 2 is always installed after the catalytic converter.

Depending on the vehicle, it may be located in the:

On inline engines:

Bank 1 is the only engine bank.

On V-style engines:

Bank 1 is the side containing cylinder number one.

Sensor 2 always refers to the downstream oxygen sensor mounted behind the catalytic converter.

Bank 1 Sensor 1 vs. Bank 1 Sensor 2

Understanding sensor locations prevents unnecessary repairs.

Bank 1 Sensor 1

Located before the catalytic converter.

Responsible for:

Bank 1 Sensor 2

Located after the catalytic converter.

Responsible for:

The upstream sensor controls fuel delivery.

The downstream sensor verifies whether the catalytic converter is doing its job.

How the Downstream Oxygen Sensor Works

Most downstream oxygen sensors are zirconia narrowband sensors.

The sensing element compares:

The difference creates a voltage signal.

Typical voltage range:

Because the catalytic converter smooths exhaust oxygen fluctuations, the downstream sensor normally changes voltage more slowly than the upstream sensor.

What Does “High Voltage” Mean?

A high-voltage signal means the PCM is receiving more voltage than expected from Bank 1 Sensor 2.

Possible causes include:

The PCM only detects that the voltage remains above its calibrated limit—it cannot immediately determine why.

How the PCM Detects P0138

The PCM continuously monitors Bank 1 Sensor 2 while specific operating conditions are met.

Typical monitoring requirements include:

If the downstream oxygen sensor voltage remains excessively high for a specified period, the PCM stores P0138.

Some manufacturers require multiple drive cycles before turning on the Check Engine Light.

Why the Downstream Oxygen Sensor Matters

Although Bank 1 Sensor 2 does not directly control injector pulse width, it performs several important emissions-related functions.

It helps the PCM verify:

Without accurate downstream oxygen sensor feedback, the PCM cannot reliably determine whether the catalytic converter is functioning properly.

Can a Rich Engine Cause P0138?

Yes.

An engine running excessively rich may cause the downstream oxygen sensor to report high voltage.

Possible causes include:

However, many P0138 cases are caused by electrical problems rather than actual rich engine operation.

Can a Short to Voltage Cause P0138?

Absolutely.

If the oxygen sensor signal wire contacts battery voltage or another powered circuit, the PCM may detect a constant high-voltage signal.

Possible causes include:

Signal circuit testing is essential before replacing the oxygen sensor.

P0138 Quick Facts

ItemInformation
DTCP0138
Generic DefinitionO2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
SystemFuel & Emissions
Sensor LocationDownstream (After Catalytic Converter)
SeverityLow to Moderate
Safe to Drive?Usually Yes
Common SymptomsCheck Engine Light, failed emissions inspection, catalyst monitor issues
Common CausesFailed downstream oxygen sensor, short to voltage, rich engine operation, damaged wiring
Typical Repair Cost$100–900+
Related CodesP0136, P0137, P0139, P0140, P0141, P0420

Common Vehicles That Experience P0138

Because P0138 is a generic OBD-II diagnostic trouble code, it may appear on nearly every gasoline-powered vehicle built since 1996.

Common manufacturers include:

Vehicles with high mileage or exhaust heat damage are more likely to develop wiring faults that contribute to P0138.

What Does P0138 Really Mean?

The wording of P0138 often causes people to assume the downstream 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 higher than expected.

The cause may involve:

Think of P0138 as the PCM saying:

“The downstream oxygen sensor voltage is staying much higher than it should, and I need to determine whether the problem is the sensor, the circuit, or the engine itself.”

Common P0138 Scan Tool Patterns

Monitoring live scan-tool data is one of the fastest ways to diagnose P0138.

Unlike the upstream oxygen sensor, Bank 1 Sensor 2 should produce a relatively stable voltage because it monitors exhaust gases after the catalytic converter.

The downstream sensor normally changes voltage slowly while the catalytic converter absorbs and releases oxygen during normal operation.

P0138 occurs when the PCM detects that the downstream oxygen sensor voltage remains abnormally high for longer than the manufacturer allows.

Pattern 1: Downstream Sensor Stays Above 0.8 Volts

The most common scan-tool pattern is a downstream oxygen sensor that remains near:

Possible causes include:

A voltage that never drops is the classic live-data pattern associated with P0138.

Pattern 2: Sensor Never Responds to Lean Conditions

Momentarily creating a lean condition should cause Bank 1 Sensor 2 voltage to decrease.

A healthy sensor should:

If voltage never decreases, inspect:

Failure to respond usually indicates either a sensor or electrical problem.

Pattern 3: Sensor Remains High After Deceleration

During closed-throttle deceleration:

If Bank 1 Sensor 2 remains fixed near 0.9 volts during deceleration, inspect:

Pattern 4: Upstream Sensor Operates Normally but Downstream Is Stuck High

Monitor:

Normal operation:

If Sensor 1 behaves normally while Sensor 2 remains fixed at high voltage, the problem is usually isolated to the downstream sensor or its circuit.

Pattern 5: Voltage Increases as Engine Warms Up

Some vehicles show:

Possible causes include:

Pattern 6: Catalyst Monitor Fails to Complete

Many scan tools display:

Because the PCM cannot trust the downstream sensor signal, catalyst monitoring may stop until the fault is repaired.

Common Symptoms of P0138

Many vehicles experience only minor drivability symptoms because Bank 1 Sensor 2 primarily monitors emissions rather than fuel delivery.

Common symptoms include:

Some vehicles drive normally despite the stored fault.

Check Engine Light

The Check Engine Light is usually the first symptom.

Most manufacturers require:

before storing P0138.

Failed Emissions Inspection

P0138 frequently causes emissions failures because:

Even if the engine feels perfectly normal, the inspection computer is considerably less forgiving.

Rich Exhaust Odor

If the engine is actually running rich, drivers may notice:

These symptoms usually indicate an engine management problem rather than the oxygen sensor itself.

Catalyst Monitor Problems

The PCM relies on Bank 1 Sensor 2 to verify catalytic converter efficiency.

When voltage remains too high:

Reduced Fuel Economy

Although the downstream sensor has only a limited effect on fuel delivery, some manufacturers use its information for adaptive fuel corrections.

Drivers may notice:

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

Is P0138 Serious?

P0138 is generally considered a low-to-moderate severity diagnostic trouble code.

The vehicle usually remains safe to drive.

However, ignoring the code may result in:

Prompt diagnosis helps prevent more expensive emissions-related repairs.

Can You Drive With P0138?

Usually yes.

Driving is generally safe if:

Repairs should be scheduled promptly if:

P0138 vs Related Trouble Codes

Understanding similar downstream oxygen sensor codes makes diagnosis much easier.

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 activity.

P0141

O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

Heater circuit failure.

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

Catalytic converter efficiency problem.

30 Common Causes of P0138

The most common causes include:

  1. Failed Bank 1 Sensor 2 oxygen sensor
  2. Signal wire shorted to battery voltage
  3. Internal oxygen sensor short
  4. Rich engine operation
  5. Leaking fuel injector
  6. Excessive fuel pressure
  7. Faulty fuel pressure regulator
  8. Stuck EVAP purge valve
  9. Faulty Mass Air Flow (MAF) sensor
  10. Contaminated oxygen sensor
  11. Oil contamination
  12. Coolant contamination
  13. Silicone contamination
  14. Damaged wiring harness
  15. Melted wiring insulation
  16. Connector corrosion
  17. Loose connector terminals
  18. Poor previous wiring repair
  19. Heater circuit failure
  20. Blown heater fuse
  21. Shared ground fault
  22. Catalytic converter damage
  23. Restricted catalytic converter
  24. Water intrusion into connector
  25. Aftermarket sensor incompatibility
  26. Exhaust system modifications
  27. PCM calibration issue
  28. High charging system voltage
  29. Damaged PCM connector
  30. PCM failure (rare)

Final Thoughts

The P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) code indicates the PCM has detected that the downstream oxygen sensor voltage has remained above the expected operating range.

Although a failed oxygen sensor is a common cause, rich engine operation, signal wire shorts, damaged wiring, connector corrosion, heater circuit faults, and catalytic converter issues can all create the same high-voltage condition.

Proper diagnosis should determine whether the problem originates in:

The next section of this guide covers:

Following a structured diagnostic process prevents unnecessary oxygen sensor replacement while restoring proper catalyst monitoring and emissions performance.

How to Diagnose the P0138 Code

Diagnosing P0138 requires determining why the Bank 1 Sensor 2 oxygen sensor is producing a voltage signal that remains higher than the manufacturer’s allowable limit.

Unlike the upstream oxygen sensor, which directly controls fuel delivery, the downstream oxygen sensor is primarily used to monitor catalytic converter efficiency and verify proper emissions system operation.

Although a failed downstream oxygen sensor is one of the most common causes of P0138, high voltage may also result from:

Proper diagnosis should always determine whether the problem is caused by the sensor, the electrical circuit, or the engine itself before replacing components.

Safety Precautions

Before beginning diagnosis:

The downstream oxygen sensor is mounted directly in the exhaust stream and may exceed 900°F (480°C) during normal operation.

Tools Required

Professional diagnosis may require:

A graphing scan tool or oscilloscope provides the clearest picture of downstream oxygen sensor activity.

Step 1 – Verify the Code

Connect the scan tool.

Record:

Monitor:

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

Step 2 – Check for Related Trouble Codes

P0138 commonly appears with:

Multiple codes often simplify diagnosis.

Code CombinationLikely Diagnosis
P0138 onlyDownstream sensor or signal circuit fault
P0138 + P0141Heater circuit problem
P0138 + P0172Rich engine condition
P0138 + P0420Catalytic converter efficiency issue
P0138 + P0136General downstream sensor circuit malfunction

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 closed-loop operation never begins, inspect:

Downstream oxygen sensor testing should always be performed during closed-loop operation.

Step 4 – Monitor Bank 1 Sensor 2 Voltage

Observe live scan-tool data.

A healthy downstream oxygen sensor should:

Watch for:

A consistently high signal is the defining characteristic of P0138.

Step 5 – Compare Upstream and Downstream Sensors

Monitor:

Under normal conditions:

If Sensor 2 remains fixed near 0.9 volts while Sensor 1 operates normally, inspect the downstream sensor and its electrical circuit.

If both sensors indicate rich operation, investigate possible fuel system problems.

Step 6 – Perform a Lean Condition Test

Momentarily create a lean condition.

A healthy downstream sensor should gradually decrease toward:

If voltage remains high:

Inspect:

Failure to respond strongly suggests an electrical fault or failed sensor.

Step 7 – Analyze Fuel Trim Data

Monitor:

Large negative fuel trims may indicate:

Large positive trims suggest:

Fuel trim analysis helps determine whether the high voltage reflects an actual rich condition or a sensor circuit problem.

Step 8 – Inspect the Oxygen Sensor

Inspect Bank 1 Sensor 2 for:

Contaminated sensors often produce inaccurate voltage readings.

Step 9 – Inspect the Electrical Connector

Disconnect the connector.

Inspect for:

Connector failures commonly cause unstable downstream oxygen sensor signals.

Step 10 – Test the Signal Circuit

Using a digital multimeter:

Back-probe the signal wire.

Inspect for:

A short to voltage is one of the most common electrical causes of P0138.

Step 11 – Verify Sensor Ground

Perform voltage-drop testing.

Inspect:

Poor grounds may create unstable or inaccurate sensor voltage.

Step 12 – Test the Heater Circuit

The downstream oxygen sensor contains an internal heater.

Inspect:

A malfunctioning heater may contribute to inaccurate downstream oxygen sensor operation.

Step 13 – Inspect the Fuel System

Because actual rich operation can trigger P0138, inspect:

Correct fuel system problems before replacing the oxygen sensor.

Step 14 – Inspect the Exhaust System

Inspect the exhaust between:

Check for:

Although exhaust leaks more commonly create lean readings, damaged exhaust components may affect downstream sensor behavior.

Step 15 – 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 16 – Inspect the Wiring Harness

Inspect the complete harness between:

Look for:

Heat damage remains one of the leading causes of downstream oxygen sensor circuit failures.

Step 17 – Review Technical Service Bulletins

Before replacing expensive components:

Review manufacturer Technical Service Bulletins.

Look for updates involving:

Software updates occasionally eliminate recurring faults.

Step 18 – Compare Bank 2 Sensor 2 (If Equipped)

On V-engine applications:

Compare:

If Bank 2 operates normally while Bank 1 remains at high voltage, the fault is likely isolated to Bank 1.

Step 19 – Review Freeze-Frame Data

Compare current live data with stored freeze-frame information.

Review:

Freeze-frame data often identifies the exact operating conditions under which P0138 was detected.

Step 20 – Confirm the Diagnosis

Before replacing any components, verify:

A complete diagnosis prevents unnecessary replacement of oxygen sensors, catalytic converters, and fuel system components.

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 normal voltage or that testing has verified an internal sensor 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 downstream oxygen sensor electrical connector.
  4. Remove the sensor using an oxygen sensor socket.
  5. Inspect the sensor threads and mounting 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 diagnostic trouble codes.
  10. Verify proper sensor operation with a scan tool.

OEM sensors generally provide the most accurate voltage characteristics and catalyst-monitor performance.

Step 22 – Repair the Signal Circuit

A short to battery voltage is one of the most common electrical causes of P0138.

Inspect the signal circuit for:

Repair wiring using:

The downstream harness is frequently exposed to extreme heat and road debris, making wiring failures common.

Step 23 – Repair Connector Problems

Disconnect the Bank 1 Sensor 2 connector.

Inspect for:

Connector damage can hold the signal voltage high even when the sensor itself is functioning normally.

Replace damaged connectors whenever reliable electrical contact cannot be restored.

Step 24 – Repair the Heater Circuit

Although P0141 is the primary heater-circuit code, heater problems may contribute to abnormal downstream oxygen sensor operation.

Inspect:

If the heater cannot bring the sensor to operating temperature, voltage readings may become inaccurate.

Repair heater circuit faults before replacing additional components.

Step 25 – Repair Rich Engine Conditions

If fuel trim analysis confirms the engine is actually running rich, repair the fuel system before replacing the oxygen sensor.

Inspect:

Correcting the rich-running condition often restores normal downstream oxygen sensor voltage.

Step 26 – Repair Ground Circuit Problems

Poor electrical grounds can create unstable oxygen sensor readings.

Perform voltage-drop testing.

Inspect:

Repair loose, corroded, or damaged grounds before replacing additional components.

Step 27 – Inspect the Catalytic Converter

Although P0138 primarily identifies a downstream sensor voltage problem, catalytic converter damage may contribute to abnormal sensor behavior.

Inspect for:

If P0420 is also present, perform catalyst efficiency testing before replacing the converter.

Step 28 – Update PCM Software

Before replacing expensive components, review manufacturer Technical Service Bulletins (TSBs).

Manufacturers occasionally release software updates addressing:

Always verify the PCM has the latest available software.

Step 29 – Verify OBD-II Readiness

After completing repairs:

Monitor readiness monitors using a professional scan tool.

Confirm successful completion of:

Incomplete readiness monitors may indicate that additional faults remain.

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 P0138

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 voltage no longer remains above approximately 0.8–0.9 volts continuously.
  7. Compare upstream and downstream sensor activity.
  8. Perform a complete road test.
  9. Verify readiness monitors complete successfully.
  10. Confirm no pending or permanent DTCs remain.

A successful repair restores proper downstream oxygen sensor operation and accurate 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
Fuel system repair$200–900
Ground circuit repair$100–300
PCM software update$100–300
Catalytic converter replacement$900–2,800+
PCM replacement$1,000–3,000+

Repair costs vary depending on vehicle make, labor rates, and component accessibility.

Common Diagnostic Mistakes

Avoid these common mistakes:

Many P0138 repairs involve correcting electrical or fuel system problems rather than replacing the oxygen sensor itself.

Manufacturer-Specific Notes

General Motors

Inspect:

Heat damage to the rear oxygen sensor harness is common on older GM trucks and SUVs.

Ford

Verify:

Turbocharged EcoBoost engines should also be inspected for fuel system problems that may contribute to rich operation.

Toyota / Lexus

Inspect:

OEM oxygen sensors are strongly recommended for accurate catalyst monitoring.

Honda / Acura

Inspect:

Poor-quality aftermarket sensors may produce incorrect voltage characteristics.

Nissan / Infiniti

Compare:

Contaminated MAF sensors are a common contributor to rich-running conditions.

Hyundai / Kia

Inspect:

Water intrusion and road salt frequently contribute to connector corrosion.

Volkswagen / Audi

Verify:

Turbocharged engines require careful inspection of fuel delivery and catalyst efficiency.

BMW / MINI

Use factory diagnostic software to monitor:

Many BMW models require manufacturer-specific test routines for accurate diagnosis.

Chrysler / Dodge / Jeep / Ram

Inspect:

High exhaust temperatures frequently damage the downstream sensor wiring.

Frequently Asked Questions

What usually fixes P0138?

The most common repairs include replacing the downstream oxygen sensor, repairing damaged wiring, correcting a short-to-voltage condition, repairing rich engine operation, replacing damaged connectors, or restoring proper grounds.

Can I drive with P0138?

Yes. Most vehicles remain drivable, but prolonged rich operation can reduce fuel economy, increase emissions, and potentially shorten catalytic converter life.

Does P0138 always mean the oxygen sensor is bad?

No. Wiring faults, rich fuel conditions, shorted signal wires, connector corrosion, heater circuit failures, and PCM-related issues can all cause P0138.

Can a rich engine cause P0138?

Absolutely. Excess fuel in the exhaust reduces oxygen levels, causing the downstream oxygen sensor to produce a consistently high voltage signal.

Can a short to battery voltage trigger P0138?

Yes. A signal wire shorted to battery voltage is one of the most common electrical causes of P0138 and can mimic a failed oxygen sensor.

Can a catalytic converter cause P0138?

Sometimes, but it is not the most common cause. Most catalytic converter failures trigger P0420, although severe converter damage may affect downstream sensor behavior.

Final Thoughts

The P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) code indicates the PCM has detected that the downstream oxygen sensor voltage has remained above its expected operating range. While a failed oxygen sensor is a common cause, rich engine operation, shorted wiring, damaged connectors, heater circuit failures, fuel system faults, and catalytic converter issues can all create the same condition.

A complete diagnosis should always verify:

Following a structured diagnostic process restores accurate catalyst monitoring, prevents unnecessary oxygen sensor replacement, improves emissions performance, and protects the catalytic converter from damage caused by prolonged rich operation.

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