When an oxygen sensor in a vehicle starts to have issues, either from a bad fuel mixture or the sensor itself going bad, it will trigger a number of codes in a vehicle computer. These then appear typically as a PO136 or P0137 code for anyone using an OBDII code scanner to diagnose an engine repair light on the dashboard.


The P0137 code is associated with the second oxygen sensor installed on a car or truck, usually after the catalytic convert to test the oxygen level after the exhaust gases have moved through the converted. When the sensor’s voltage level stays a below standard expected level for longer than 120 minutes, it will send a trigger signal which the car’s computer then interprets as a potential repair issue. Ergo the repair light turning on.

Because the rear O2 sensor and related solenoid get less activity than the one before the catalytic converter, it’s more prone to either throwing a true exhaust mix failure or sensor failure code and less other reasons, such as wear and tear. This situation can also lead to a signal being triggered when the driver isn’t necessarily realizing any traditional O2 symptoms, such as a rough idle, misfiring, or similar air-fuel mixture problems.

Causes and Symptoms

  • The causes for a rear O2 sensor trigger can and often do include the following:
  • A plugged catalytic converter needing replacement
  • A bad O2 sensor or failing solenoid
  • Engine running rich and giving off more fuel gases than oxygen
  • Engine running very lean and then becomes rich in idle
  • Misfiring of the engine due to timing problems
  • Extreme fuel pressure fluctuation
  • Bad signal resistance between the sensor and car’s computer


Diagnosis starts with confirming the sensor is working properly first as this is often the cheapest option with the least amount of work involved. Sensors tend to go bad after a significant amount of time and use. If no apparent engine problem signs exist, such as misfiring or belching smoke from the exhaust or the car failing an in-shop exhaust test, then the P0137 flag should be cleared and the car driven again to repeat the code trigger, if possible. A slightly poor fuel mix won’t result in a critical engine failure, but it will eventually trigger the sensor again even when replaced with a new one.

Where the signs of a bad air-fuel mixture are apparent, especially in the vehicle’s regular operating condition, then the fuel mixture should be adjusted correctly and the sensor should be replaced with a new one. Some turbo engines will trigger both issues, and the problem frequently has to do with a dirty throttle intake valve. This valve part gets gummed up quickly when a turbo engine is used for regular commuting instead of higher temperature driving. No surprise, the gunk and poor performance then triggers a “running rich” problem at idle, which can throw a number of signals to the computer’s code, including O2 sensor problems.