Code P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

What Does the P0138 Code Mean?

Diagnostic Trouble Code (DTC) P0138 is defined as “O2 Sensor Circuit High Voltage (Bank 1 Sensor 2).”

Your car’s computer, Powertrain Control Module (PCM), sets this code when it detects that the voltage signal from the rear heated oxygen sensor (HO2S) is too high for a given period of time. 

Why does this happen?
Most modern cars feature two O2 sensors (oxygen sensors) that help assess the amount of oxygen in the exhaust stream. The O2 sensor 2 is a downstream oxygen sensor or the secondary heated oxygen sensor (HO2S 2) on Bank #1.

Bank 1 is the side of the engine that houses cylinder number 1. 

Sensor 2 monitors the efficiency of the catalytic converter by measuring oxygen levels in the exhaust stream after it leaves the converter. Under normal conditions, it sends a steady, low-voltage signal to the PCM.

However, when the PCM detects a high voltage signal (0.7V to 1.0V) from the downstream O2 sensor circuit for an extended period, it interprets it as a possible rich fuel condition, sensor fault, or exhaust issue. As a result, the PCM triggers the P0138 code.

What are the Possible Causes of P0138?

Here are the common triggers for the P0138 code’s high voltage signal:

• Malfunctioning or damaged O2 sensor (or an old sensor with more than 100,000 miles)
• O2 sensor low control circuit is open
• O2 sensor heater element is compromised
• Short circuit in the O2 sensor circuit
• High battery voltage
• Corroded harness or damaged wiring
• High fuel pressure (or a defective fuel pressure regulator)
• Fuel injector leaks or engine vacuum leaks
• Ignition misfires
• Failing engine coolant temperature sensor
• Mass airflow sensor failure (MAF sensor)
• Manifold air pressure sensor failure (MAP sensor)
• Inefficient or malfunctioning catalytic converter (rare cases)
• Powertrain control module issues (rare cases)

6 Common Symptoms of P0138 to Watch Out For

Here’s what you may experience when your vehicle has a high-voltage HO2S signal: 

1. Blinking check engine light: When the engine control module stores error code P0138, you’ll immediately notice an active check engine light or malfunction indicator light.

2. Engine running rich: If your engine is running rich, it has excess fuel (less oxygen), which leads to low fuel economy and incomplete combustion. This can result in engine performance problems.

3. Rough idling or stalling: The engine may vibrate heavily due to combustion of an imbalanced air-fuel mixture, resulting in rough idling. Moreover, to compensate for high fuel pressure, the engine may run lean (with a high air-fuel ratio) during sensor testing — leading to misfires or stalling.

4. Lack of acceleration: A faulty downstream O2 sensor may falsely signal a rich fuel mixture, causing the PCM to adjust fuel delivery incorrectly. This can lead to poor combustion, reduced engine power, and sluggish throttle response, especially during acceleration.

5. Heavy exhaust fumes: When you’re driving, or even if the car is idle, you’ll notice heavy exhaust gas emissions. This is because a faulty O2 sensor can lead to an improper air-fuel ratio and incomplete fuel combustion. These emissions will also have a strong odor.

6. Failed emissions test: As a result, your vehicle is likely to fail an emissions test due to the heavy exhaust fumes releasing excessive pollutants into the air.

Can You Still Drive with Code P0138?

While it’s possible to drive with diagnostic trouble code P0138, it’s best to avoid doing so once you detect the code on your scan tool. 

P0138 is a moderately severe trouble code, and extended driving under a rich condition can lead to: 

• Fouled spark plugs
• Internal engine damage
• Failure of the catalytic converter and related components
• Reduced fuel efficiency 
• More pollutants in emissions

Additionally, you won’t be able to tell if other engine light codes are popping up while you leave this issue unchecked. So, addressing the check engine light as soon as possible could help save your vehicle from further damage and costly repairs. 

How to Diagnose Trouble Code P0138: A General Know-How

Code P0138 could result from several issues. So, it’s best to let a mechanic conduct a diagnosis before you replace any expensive parts:

Here’s how a mechanic would troubleshoot a P0138 issue:

1. Check the codes using a scan tool, document the freeze frame data, and clear the codes to verify the issue.

2. Check the bank 1 sensor 2 for faulty wiring, a loose connector, or wiring harness corrosion.

3. Monitor O2 sensor voltage live data. A consistently high voltage (0.7V to 1.0V) on sensor 2 indicates the code’s condition.

4. Compare the voltage readings with sensor 1 — while voltages won’t be identical (sensor 1 fluctuates, sensor 2 should be stable), sensor 2 should not read constantly rich.

5. Check the O2 sensor for physical damage or signs of oil, coolant, or fuel contamination from engine leaks.

6. Test the engine coolant temperature sensor to see if it’s causing a rich condition.

7. Test the fuel pressure regulator to ensure it meets manufacturer specifications and to check for possible high fuel pressure (a low air-fuel ratio).

8. Check for exhaust leaks before the downstream O2 sensor, a loose gas cap, and a clogged or damaged catalytic converter.

9. Follow other pinpoint tests for bank 1 sensor 2 issues specified in the manufacturer’s manual.

4 Common Mistakes to Avoid While Diagnosing P0138

If you plan to take the DIY route for fixing code P0138, here are a few mistakes to keep in mind: 

1. Immediately replacing the O2 sensor without testing it: Sometimes, it’s not a faulty oxygen sensor — the high voltage could be caused by fuel, wiring, or exhaust issues. Test related components before replacing.

2. Ignoring wiring and connector checks: Damaged wiring, corroded pins, or loose connectors can mimic sensor failure. Inspect the harness and connectors before replacing the sensor.

3. Not clearing the codes and retesting after repairs: Always clear the DTC and drive the vehicle to see if the same code or related codes return before finalizing repairs.

4. Using a generic OBD II scanner without live data capability: Live voltage readings are crucial for accurately diagnosing P0138. Basic code readers may miss important data.

How to Fix DTC P0138 (+ Repair Costs)

Here are the possible repairs your mechanic would perform to fix DTC P0138:

1. They’ll replace a faulty wiring harness and damaged, corroded, or loose electrical parts.
2. Repair the O2 sensor signal circuit in case of a short due to high battery voltage. 
3. Replace the engine coolant temperature, MAP, or MAF sensors if their output signal falls outside expected parameters. 
4. Replace the faulty oxygen sensor (rear sensor) on Bank 1 along with the upstream sensor (it’s best to replace them in pairs). 
5. Replace a leaking fuel injector or a bad fuel pressure regulator.
6. Replace a clogged or damaged catalytic converter. 
7. Repair any exhaust leaks, if present. 

Once the repairs are done, your mechanic will clear the code using the OBD II scanner, verify whether the check engine light is still flashing, and test-drive the car to ensure smooth performance.

So, how much do these repairs cost?

A mechanic usually charges a flat diagnostic fee between $20 and $500. The final repair estimate will depend on the diagnosis, your mechanic’s labor rate, and the repairs needed.

Here are the estimated costs (excluding labor) for fixing the P0138 trouble code: 

• Oxygen sensor wiring replacement: $20-$60
• Engine coolant temperature sensor replacement: $300-$325
• MAP sensor replacement: $30-$70
• MAF sensor replacement: $200-$300
• Oxygen sensor replacement (both front and rear sensors): $200-$300
• Fuel pressure regulator replacement: $160-$650
• Catalytic converter replacement: $300-$4000

How to Prevent P0138 Trouble Code from Recurring

Here’s what you can do to prevent the P0138 error code from popping up:

• Keep up with regular oil changes to avoid sensor contamination.
• Insulate upstream and downstream sensor wires to prevent corrosion.
• Address misfires and exhaust leaks quickly.
• Use quality fuel to reduce buildup in the catalytic converter.