Quick answer: The P0138 code indicates that the powertrain control module has detected a high voltage output from the rear oxygen sensor, specifically sensor 2 on bank 1. This affects the emissions control system. It is moderately urgent, meaning you should schedule a diagnostic check soon to avoid catalytic converter damage.
Seeing a Check Engine light on your dashboard is rarely a welcome sight, and discovering an OBD-II code like P0138 can leave you wondering what needs fixing. This article breaks down what this diagnostic trouble code means for your car, how to spot the symptoms, and the smartest ways to approach the repair.
What Does P0138 Mean?
Your vehicle relies on oxygen sensors to measure unburned oxygen in the exhaust stream, helping the computer adjust the air-fuel mixture. Code P0138 specifically points to Bank 1, Sensor 2, which is the downstream sensor located after the catalytic converter. A “high voltage” reading means the sensor is sending a signal above normal operating parameters, usually staying stuck above 1.0 volt for too long. This tells the engine computer that the exhaust has a continuous lack of oxygen, or more commonly, that the sensor circuit has an electrical short to voltage.
Symptoms
- Check Engine light illuminated on the dashboard
- Noticeably reduced fuel economy
- Engine running rough or hesitating during acceleration
- Black smoke or a strong sulfur smell from the exhaust pipe in rare cases
- Failure during a mandatory emissions inspection or smog test
Possible Causes
Before throwing parts at the problem, remember that the most common mistake is replacing the most expensive part first. Work through these potential culprits in order.
- Faulty downstream oxygen sensor with an internal electrical short
- Damaged, melted, or frayed wiring harness leading to the oxygen sensor
- Corroded or loose electrical connector at the sensor plug
- Exhaust leak near the rear oxygen sensor allowing outside air interference
- Engine running extremely rich due to a failing fuel injector or faulty fuel pressure regulator
- Internal failure within the engine control module
Check These Yourself Before Visiting a Mechanic
- Visually inspect the wiring harness and connector near the exhaust pipe for signs of melting, hanging loose, or rubbing against hot components.
- Look under the car with a flashlight for obvious exhaust leaks, cracks, or loose gaskets around the catalytic converter and downstream sensor area.
How to Fix It
- Clear the trouble code using an OBD-II scanner and inspect live data to see if the voltage drops or stays permanently high.
- Repair any damaged wiring or clean corrosion out of the sensor electrical connector.
- Fix any exhaust leaks upstream or around the sensor mounting bung that skew oxygen readings.
- Replace the downstream oxygen sensor if wiring and exhaust integrity check out normal.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Oxygen Sensor Wiring Repair | 10 – 50 | 75 – 150 |
| Exhaust Leak Repair | 20 – 100 | 100 – 250 |
| Downstream Oxygen Sensor Replacement | 60 – 200 | 75 – 150 |
Note: these are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0138 code?
Yes, you can usually drive short distances because this code involves the downstream sensor, which primarily monitors emissions performance rather than core engine operation. However, driving for too long with a rich running condition can prematurely clog or ruin your expensive catalytic converter.
How do I clear the P0138 code?
You can clear the code using a standard OBD-II scanner plugged into your car’s diagnostic port under the dashboard. Keep in mind that if the underlying mechanical or electrical fault is not actually fixed, the Check Engine light and code P0138 will return shortly after driving.
Will a P0138 code cause my car to fail an emissions test?
Yes, any active powertrain diagnostic trouble code will cause an automatic failure during a state emissions inspection or smog check. Even if your car runs fine, the illuminated Check Engine light prevents passing.