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P codepowertrainMedium severity

P067BCylinder 4 Glow Plug Control Circuit High

The control circuit voltage for the cylinder 4 glow plug is stuck above the expected range, typically from a short to battery voltage or a failed module driver.

Definition

The glow plug control module drives each plug through a dedicated control circuit and monitors the feedback voltage against commanded state. P067B sets when the cylinder 4 control circuit reads high when it should be low or off. Depending on the failure, the plug may never energize (hard cold starts, white smoke, MIL) or may stay energized continuously, which overheats the plug; a plug that overheats can swell or shed its tip into the cylinder, so the code should not be ignored. Warm-start behavior is usually normal, which can mask the fault.

Common causes

  • Cylinder 4 control wire shorted to battery voltage
  • Failed (stuck-on or open) driver in the glow plug control module
  • Internally shorted glow plug pulling the circuit out of range
  • Corrosion or water intrusion in the harness connector biasing the circuit
  • Chafed harness contacting a powered circuit

Common misdiagnoses

  • Replacing all glow plugs when the fault is in the wiring or module
  • Replacing the glow plug control module before checking for a harness short
  • Battery or alternator diagnosis for the voltage-high condition
  • PCM replacement

Troubleshooting steps

  1. 1.Record codes and note companions

    Check whether the low-side sibling P067A or other cylinders are also flagged, which points toward the module or a shared harness section.

  2. 2.Measure voltage at the cylinder 4 control circuit

    With the glow system commanded off, battery voltage on the control wire confirms a short to power or a stuck-on driver.

  3. 3.Isolate the harness

    Disconnect the module and the plug; if voltage on the wire persists, find and repair the short to a powered circuit.

  4. 4.Test the glow plug itself

    Compare the cylinder 4 plug's resistance to a known-good cylinder — an internal short reads well below spec.

  5. 5.Verify module operation

    With good wiring and a good plug, a circuit that stays high at the module pin confirms a failed glow plug control module.

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