P codes have (incorrectly) come to be associated with Powertrain fault codes used in OBDII.

In reality, P codes are PIDs, or "Parameter IDs", which are codes used to request data from a vehicle.

As well as P codes, there are also B, C and U codes. These codes identify which control system set a particular fault:

  • B - Body
  • C - Chassis
  • P - Powertrain
  • U - Network Communication

With Powertrain Codes, P0, P2, P3, and U0 are generic codes and P1, P3, and U1 codes are manufacturer specific

P - Powertrain (Complete list of codes)

More about P codes from Wikipedia

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