P0143 - OBD-II Trouble Code

Auto Systems and Repair

Oxygen Sensor Circuit Low Voltage (Bank 1, Sensor 3)

Our emissions expert has put together the following information about the P0143 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 P0143

Fault Code Definition
The Rear Catalyst Monitoring Oxygen Sensor evaluates the rear most Catalytic Converter's emissions-reducing capability in a system with two Catalytic Converters for each bank of cylinders. P0143 is set when the Rear Oxygen Sensor voltage signal remains below 200 millivolts for an extended period of time.


  • 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 many cases, the NOx emissions will be elevated

Common Problems That Trigger the P0143 Code

  • Defective Rear Most Oxygen Sensor 
  • Faulty Rear Most Oxygen Sensor wiring or connections

Common Misdiagnoses

  • Oxygen Sensor is replaced when the problem is faulty sensor wiring or connections
  • Rear Catalytic Converter is replaced when the problem is an exhaust leak before the Rear Oxygen Sensor
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