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P codepowertrainMedium severity

P066ECylinder 3 Glow Plug Control Circuit Low

The control circuit for the cylinder 3 glow plug is stuck at a lower voltage than expected, usually meaning the circuit is shorted to ground.

Definition

The glow plug control module monitors each plug's circuit voltage while switching it, and the PCM sets P066E when the cylinder 3 circuit reads low when it should be high — the signature of a short to ground in the feed wire, a shorted glow plug, or a stuck-on driver in the module. A grounded circuit can blow the glow plug fuse or overheat the wiring, and a plug held on continuously can burn out or, in the worst case, suffer tip damage inside the cylinder. Cold-start symptoms (long cranking, white smoke, rough running) appear if the circuit protection has opened, and the MIL or glow plug lamp illuminates.

Common causes

  • Cylinder 3 glow plug feed wire chafed and shorted to ground
  • Internally shorted cylinder 3 glow plug
  • Melted or oil-soaked harness/connector at the cylinder head
  • Failed glow plug control module driver stuck on or shorted low
  • Corrosion bridging the plug terminal to the head

Common misdiagnoses

  • Replacing the full glow plug set for a single-circuit wiring short
  • Replacing the glow plug control module before finding a grounded harness wire
  • Replacing glow plug fuses repeatedly without locating the short
  • Blaming the battery or starter for cold-start complaints

Troubleshooting steps

  1. 1.Check glow plug circuit fuses

    A blown feed fuse corroborates a short to ground and tells you the circuit has been dead for cold starts.

  2. 2.Isolate the plug from the harness

    Disconnect the cylinder 3 lead; if the code re-sets with the plug disconnected, the short is in the wiring or module side, not the plug.

  3. 3.Measure the plug for an internal short

    Resistance well below the sibling plugs (near 0 ohms where spec is 0.5-2 ohms) or signs of overheating at the terminal condemn the plug.

  4. 4.Test the feed wire for a short to ground

    With both ends disconnected, an ohmmeter from the wire to chassis ground should read open; continuity means finding and repairing the chafe point, commonly where the harness crosses the head or valve cover.

  5. 5.Verify the module isn't holding the circuit low

    With wiring and plug proven good, back-probe the module output during a pre-glow cycle; an output that never switches high indicates a failed control module.

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