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DTC codes / P06xx · Computer and output circuits / P068C

P068CCylinder 7 Glow Plug Control Circuit Low

P068C means the engine control module (ECM) commanded cylinder 7's glow plug on but detected the control circuit voltage staying too low, indicating a short to ground or excessive current draw. The ECM monitors the glow plug control circuit for expected voltage behavior and flags this code when the circuit cannot reach the commanded state.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Illuminated check engine light, often with no other noticeable symptoms
  • Hard cold starting or rough idle immediately after a cold start
  • Extended cranking time before the engine fires in cold weather
  • White smoke from the exhaust during cold running
  • Misfire or rough running during the warm-up period
  • Possible glow plug-related codes stored alongside P068C

Common causes, most likely first

  1. Failed glow plug with an internal short to ground
  2. Chafed or shorted glow plug wiring harness near the cylinder 7 glow plug
  3. Corroded or loose electrical connector at the cylinder 7 glow plug
  4. Faulty glow plug control module (GPCM) output driver for cylinder 7
  5. Short to ground in the control circuit between the GPCM and the glow plug
  6. Water or oil intrusion into the glow plug connector or harness
  7. Damaged glow plug boot or insulator allowing the terminal to contact ground

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data to confirm P068C is active and not history
  2. Visually inspect the cylinder 7 glow plug connector and harness for chafing, corrosion, or contamination
  3. Measure glow plug resistance at the cylinder 7 glow plug and compare it to the manufacturer's specification
  4. Disconnect the glow plug and check the control circuit for a short to ground with a multimeter
  5. Inspect the glow plug control module connector and wiring for damage or corrosion
  6. If the circuit tests good, substitute a known-good glow plug or control module output to isolate the fault
  7. Clear codes and perform a cold-start verification drive to confirm the repair

Glow plug circuit shorts that fool technicians

  • A glow plug with normal resistance when cold can still short internally once it heats up, so a cold ohm check alone may miss the fault.
  • The glow plug control module often shares a common ground with other circuits, and a bad ground can mimic a cylinder-specific short.
  • Harness chafing near the cylinder head or exhaust manifold is a common cause that only appears after the engine reaches operating temperature.
  • P068C can be set by a control module software issue on some vehicles, so check for technical service bulletins before replacing parts.
  • A shorted glow plug can draw enough current to damage the control module output driver, so always recheck the module after replacing a shorted plug.

Can I keep driving with P068C?

You can usually drive the vehicle, but cold starting will be difficult and prolonged cranking can stress the starter and battery. Avoid heavy electrical loads during cold starts and repair the fault before winter temperatures drop further.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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