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P codepowertrainMedium severity

P0372Timing Reference High Resolution Signal "A" Too Few Pulses

The PCM is counting fewer high-resolution timing pulses per crankshaft revolution than the system should produce.

Definition

The PCM verifies the high-resolution timing reference by counting its pulses against each crankshaft revolution measured on the low-resolution signal. P0372 sets when pulses are missing — the counted total falls short of the design count — indicating signal dropouts from a weak sensor, a contaminated optical pickup, or an intermittent circuit. Drivers typically notice stumbling, surging, or stalling (especially in damp conditions on optical-distributor engines), possible hard starting, and a MIL; sustained signal loss degrades spark accuracy and misfire detection.

Common causes

  • Moisture or oil contamination inside an optical distributor blocking pulse slots
  • Weak or failing high-resolution sensor dropping pulses
  • Intermittent open in the signal wiring or a loose connector terminal
  • Debris or damage on the trigger disc obscuring slots or teeth
  • Excessive sensor air gap or reluctor runout
  • Poor sensor ground causing marginal signal amplitude

Common misdiagnoses

  • Fuel pump replaced for stalling actually caused by signal dropout
  • Ignition coil replaced when the timing reference is at fault
  • PCM replaced before scoping the pulse train
  • Repeated tune-ups performed without addressing distributor contamination

Troubleshooting steps

  1. 1.Capture the pulse train on a scope

    Compare the high-resolution channel against the low-resolution reference while cranking and at idle; look for gaps where pulses disappear.

  2. 2.Inspect the optical distributor or sensor

    On Opti-Spark-style units, check for water intrusion and oil from the front cover seal; on sensor systems, check the air gap and tip condition.

  3. 3.Wiggle-test the circuit

    Flex the harness and connector while watching the scope; dropouts during manipulation localize an intermittent open.

  4. 4.Check connector terminals

    Verify terminal tension and look for corrosion at the sensor/distributor connector and the PCM connector.

  5. 5.Repair the source and reverify

    After fixing the seal leak, sensor, or wiring, confirm a complete, gap-free pulse count at idle and under load before returning the vehicle.

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