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P codepowertrainHigh severity

P0374Timing Reference High Resolution Signal "A" No Pulses

The PCM sees the engine turning on the low-resolution reference but is receiving no pulses at all from the high-resolution timing signal.

Definition

The PCM cross-checks its two timing references: when the low-resolution crankshaft signal shows the engine rotating but zero pulses arrive on the high-resolution circuit, P0374 sets. This is a complete signal failure — a dead sensor or optical pickup, a severed circuit, or lost power to the sensor. Depending on the system's fallback strategy the engine may start and run roughly on the low-resolution signal alone or may stall and refuse to start, and the MIL will illuminate; on GM optical-distributor (Opti-Spark) applications this code frequently accompanies a no-start.

Common causes

  • Failed optical sensor/distributor pickup (moisture or oil ingress is common)
  • Open circuit in the high-resolution signal wire
  • Disconnected, corroded, or damaged connector
  • Loss of power or ground to the sensor/distributor
  • Failed high-resolution crank/cam sensor on non-distributor systems
  • PCM input failure (least common)

Common misdiagnoses

  • Fuel pump replaced for a no-start caused by the dead timing signal
  • Ignition coil replaced when the trigger signal is absent
  • PCM replaced before checking sensor power, ground, and continuity
  • Battery and starter serviced for what is a run/stall fault

Troubleshooting steps

  1. 1.Confirm the condition on a scope

    While cranking, verify the low-resolution signal is present and the high-resolution channel is completely flat — this distinguishes P0374 from a miscount fault.

  2. 2.Check sensor power and ground

    Verify the specified supply voltage and a good ground at the sensor or distributor connector with the key on.

  3. 3.Test circuit continuity to the PCM

    With both ends disconnected, confirm under 1 ohm on the signal wire end to end and no short to ground or power.

  4. 4.Inspect the sensor or distributor internally

    On optical units, look for water, oil, or corrosion on the optics and trigger disc; contamination or a dead pickup means replacement.

  5. 5.Replace the failed component and verify

    After repair, confirm a full-amplitude, continuous pulse train while cranking and at idle, then clear codes and road test.

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