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P codepowertrainMedium severity

P066BCylinder 1 Glow Plug Control Circuit High

The control circuit for the cylinder 1 glow plug is showing a higher voltage than expected, pointing to a short to power or an open circuit on that plug's feed.

Definition

On diesel engines with individually controlled glow plugs, the glow plug control module switches and monitors each plug's circuit and reports faults to the PCM. P066B sets when the cylinder 1 circuit voltage stays high when it should be pulled low — typically an open glow plug, a broken wire, or a short to battery voltage on that leg. Expect hard starting, extended cranking, white-blue smoke, and rough running in cold weather, with little or no symptom when warm; the MIL or glow plug lamp illuminates. On modern diesels that also use glow plugs for regeneration or combustion stability, emissions performance can suffer.

Common causes

  • Failed (open) cylinder 1 glow plug element
  • Broken wire or corroded terminal between the glow plug control module and the cylinder 1 plug
  • Cylinder 1 circuit chafed and shorted to battery voltage
  • Loose or carbon-fouled glow plug terminal nut/connector at the plug
  • Faulty glow plug control module output stage

Common misdiagnoses

  • Replacing the entire glow plug set when only cylinder 1's circuit is faulted
  • Replacing the glow plug control module when the plug itself is open
  • Replacing the battery or starter for a hard-cold-start complaint
  • Replacing fuel injectors to chase cold-start white smoke

Troubleshooting steps

  1. 1.Measure cylinder 1 glow plug resistance

    Disconnect the plug lead and measure element-to-block; expect roughly 0.5-2 ohms for low-voltage ceramic/metal plugs (up to ~6 ohms on some designs), with an open (OL) reading condemning the plug.

  2. 2.Compare against a known-good cylinder

    Measure a non-faulted cylinder's plug on the same engine; the readings should match within a few tenths of an ohm.

  3. 3.Inspect the cylinder 1 circuit wiring

    Check the lead from the control module for chafing against the head or valve cover and verify the terminal at the plug is tight and free of corrosion.

  4. 4.Verify commanded output at the plug

    During a cold-start pre-glow cycle (or scan-tool glow plug activation), confirm the expected supply voltage arrives at the cylinder 1 terminal; voltage present with a good plug shifts suspicion back to connections.

  5. 5.Test the glow plug control module output

    With a known-good plug and verified wiring, a circuit that still won't pull low indicates a failed module output channel; replace the module.

  6. 6.Use anti-seize discipline when replacing plugs

    Torque the new plug to spec (typically 8-15 Nm depending on engine) — overtightening risks snapping the plug in the head.

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