Code 12 - No Reference Pulse
Code 12 means ECM is on, but sees no reference pulse from Engine Speed Sensor (normal condition with ignition on and engine not running). Code 12 does not store, and will only flash when fault is present. With engine running, Code 12 could mean an open or ground in sensor reference circuit.
The sensor receives a 5-volt reference signal and pulses this signal back to the ECM 4 times per revolution. The ECM measures engine RPM by counting these pulses.
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- Checks for a good 5-volt reference signal.
- Checks for proper ECM voltage to Engine Speed Sensor. If circuit to ECM is complete, normal voltage will be about 5 volts with harness disconnected from sensor.
- Checks for good sensor ground circuit (452) to ECM. Indicates whether open indicated in step 2) is in wire or at ECM.