GM OBD Trouble Code 12

No Distributor Reference Pulse (Diagnostic Test Active)

The Engine Control Module uses the distributor reference pulse to calculate the engine speed. On newer engines, the Crank Shaft Position Sensor performs this function.

Fault Code Description

12 - No Distributor reference pulse (diagnostic test active)


  • When flashing the Check Engine Light to read codes, code 12 will always display first, indicating the diagnostic system is active
  • The engine will not run with a "hard" code 12 set

Common Problems

  • Chances are you will never need to diagnose a code 12. Rather, the problem would be a crank, no-start condition.
  • If code 12 does not flash, that would indicate a communication problem with the ECM.
  • Code 12 does not show when reading codes with a scan tool. If there is a communication problem, the scan tool will let you know.

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