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P codepowertrainMedium severity

P066DCylinder 2 Glow Plug Control Circuit High

The control circuit for the cylinder 2 glow plug is showing a higher voltage than expected, indicating an open plug, broken wiring, or a short to power on that circuit.

Definition

The glow plug control module individually drives each glow plug and monitors circuit voltage for feedback; the PCM stores P066D when the cylinder 2 circuit reads high when it should be switched low. The usual culprit is an open glow plug element or a break in that cylinder's feed wire, with a short to battery voltage as the less common cause. Symptoms concentrate in cold weather: long crank times, white smoke, misfire-like roughness until the engine warms, and an illuminated MIL or glow plug indicator. Diesels that fire glow plugs during DPF regeneration or light-load operation may also show reduced emissions performance.

Common causes

  • Failed (open) cylinder 2 glow plug element
  • Broken or corroded wiring between the glow plug control module and cylinder 2
  • Cylinder 2 feed wire shorted to battery voltage
  • Loose terminal nut or oil-soaked connector at the glow plug
  • Faulty glow plug control module driver channel

Common misdiagnoses

  • Replacing all glow plugs when only cylinder 2's circuit is at fault
  • Replacing the glow plug control module before testing the plug's resistance
  • Replacing batteries for slow cold starting
  • Chasing fuel-system or injector faults for cold-start smoke

Troubleshooting steps

  1. 1.Measure cylinder 2 glow plug resistance

    With the lead disconnected, expect roughly 0.5-2 ohms to ground (design-dependent); OL confirms an open element.

  2. 2.Cross-check a non-faulted plug

    A healthy sibling cylinder on the same engine gives the correct baseline reading to compare against.

  3. 3.Inspect and wiggle-test the cylinder 2 lead

    Trace the wire from the module for heat damage, chafe points, and a loose or corroded terminal at the plug, wiggling during a monitored pre-glow cycle to catch intermittents.

  4. 4.Confirm voltage delivery during pre-glow

    Command the glow plugs with a scan tool or a cold start and verify system voltage (typically 5-11 V on modern low-voltage plugs, per spec) appears at the cylinder 2 terminal.

  5. 5.Condemn the control module last

    Only after the plug tests good and the wiring shows continuity with no short to power should the glow plug control module be replaced.

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