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P codepowertrainMedium severity

P0373Timing Reference High Resolution Signal "A" Intermittent/Erratic Pulses

The high-resolution engine timing signal is cutting in and out intermittently or arriving as chaotic, erratic pulses.

Definition

The PCM expects the high-resolution timing reference to arrive as a steady, evenly spaced pulse train synchronized to crankshaft rotation. P0373 sets when the signal is intermittent or erratic — bursts of pulses, irregular spacing, or momentary losses — rather than cleanly missing or miscounted. Because the PCM leans on this signal for fine spark timing and misfire detection, symptoms include random stumbles, surging, intermittent stalling that may be weather- or heat-dependent, hard hot restarts, and a MIL.

Common causes

  • Intermittent connection in the signal circuit (loose terminal or chafed wire)
  • Heat- or moisture-sensitive failing sensor or optical pickup
  • Condensation cycling inside an optical distributor
  • Damaged circuit shielding letting ignition noise corrupt the signal intermittently
  • Excessive reluctor/trigger disc runout causing amplitude variation
  • Marginal sensor ground connection

Common misdiagnoses

  • Ignition module or coil replaced for intermittent stalling
  • Fuel pump replaced when the stall is signal-related
  • PCM replaced for a harness intermittent
  • Sensor replaced without finding the chafed or loose wiring that caused the symptom

Troubleshooting steps

  1. 1.Correlate the symptom with conditions

    Note whether stalls happen in rain, at operating temperature, or over bumps — each pattern points to moisture, thermal, or mechanical/wiring causes respectively.

  2. 2.Monitor the signal during a road test

    Record the high-resolution channel with a scope or graphing meter while driving until the stumble occurs; confirm the signal drops when the symptom appears.

  3. 3.Wiggle- and tap-test components

    At idle, flex the harness and lightly tap the sensor/distributor while watching the trace; erratic output during manipulation isolates the fault.

  4. 4.Inspect shielding and routing

    Verify the signal circuit's shield is intact and the wiring is not routed against plug wires or the alternator.

  5. 5.Thermally test the sensor

    Warm the sensor or distributor with a heat gun while scoping; erratic pulses when hot condemn the component.

  6. 6.Verify after repair

    Reproduce the original failure conditions and confirm the pulse train stays continuous with no stored codes.

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