Code 12 indicates that the ECM is on and sees no reference pulse from the engine speed sensor. Code display is normal with ignition on and engine not running. Code 12 is not stored and will only flash when fault is present. With engine running, Code 12 could mean an open or ground in engine speed sensor reference circuit.

The engine speed sensor is a camshaft driven pick-up, mounted at center rear of engine. Sensor is sourced by a 5-volt reference voltage and allows the ECM to measure engine RPM by the number of times voltage is pulsed. The engine speed sensor pulses 4 times per revolution.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) This tests for a good 5-volt reference. Normally, the ECM should be at about 5 volts for fully charged batteries.

Test Number 2) This tests for proper ECM voltage to the engine speed sensor. If the circuit to the ECM is complete, normal voltage will be about 5 volts with harness disconnected from sensor.

Test Number 3) This tests for good sensor ground circuit No. 452, from sensor to ECM. Since test 2) indicated an open, the results of this test indicates whether the open is in the wire or at the ECM.

Fig 1: Code 12: Circuit Diagram & Flow Chart No Reference Pulse

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code 12, No Reference Pulse

Use the Manual With the Right Hardware

When to See a Mechanic