P0143 - OBD-II Trouble 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