P0139 - OBD-II Trouble Code

Auto Systems and Repair

Oxygen Sensor Circuit Slow Response (Bank 1, Sensor 2)

Our emissions expert has put together the following information about the P0139 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0139

Fault Code Definition
When the up and down "cross-counting" voltage activity from the Oxygen Sensor is too slow, code P0139 is triggered and the Check Engine/Service Engine Soon Light will illuminate.


  • Check Engine Light will illuminate
  • In many cases, no abnormal symptoms may be noticed
  • In some cases, the vehicle will fail an Emissions Test
  • In some cases, the engine may "ping" and produce excess NOx 

Common Problems That Trigger the P0139 Code

  • Defective Catalytic Converter
  • Faulty Rear Oxygen Sensor wiring or connections

Common Misdiagnoses

  • Oxygen Sensor Sensor is replaced when the problem is a defective Catalytic Converter
  • Oxygen Sensor is replaced when the problem is faulty sensor wiring or connections
  • Oxygen Sensor is replaced when the problem is an exhaust leak in front of the Oxygen Sensor
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