O2 Sensor Type 2 Signal High
The upstream oxygen sensor signal is too high indicating rich exhaust.
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What it means (plain English)
The PCM sees a higher voltage (over 1.0 volts on a narrowband sensor) from the second oxygen sensor, which means it thinks there's too much fuel in the exhaust compared to the air.
What the computer is actually seeing
The PCM monitors the voltage from the second O2 sensor (usually near the catalyst). It expects voltages to oscillate between 0.1 volts (lean) and 0.9 volts (rich). If it sees continuous high voltage above 1.0 volts, it triggers this code.
What a healthy reading looks like
O2 sensor voltage should be between 0.1V (lean) and 0.9V (rich).
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check the O2 sensor wire harness for any damage or corrosion. Repair any issues found. Decide: Is the harness okay? (Yes - proceed to step 2, No - repair wiring).
- 22. Use a scan tool to monitor O2 sensor voltages during idle and acceleration. Note the values. Decide: Do you see constant high voltage? (Yes - replace the O2 sensor, No - proceed to step 3).
- 33. Inspect for exhaust leaks before the sensor. Look for cracks or loose connections. Decide: Are there leaks present? (Yes - repair leaks, No - proceed to step 4).
- 44. Clear the code and take the vehicle for a test drive. Monitor the O2 sensor values. Decide: Did the code return? (Yes - replace the PCM, No - issue resolved).
Common causes
- Faulty upstream oxygen sensor
- Exhaust leaks before the sensor
- Wiring problems (shorts, damage)
- PCM software issue
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
Yes, but performance may be affected. It’s best to fix it soon to avoid excessive fuel use or catalyst damage.
Will this code come back after clearing it?
It can if the underlying issue isn't fixed. Monitor the sensor readings after repairs.
Why does this code appear with P0137?
P0138 signals a similar high voltage issue, often due to the same problems with the O2 sensor or its circuit.
Working a real vehicle right now?
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