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DTC codes / P06xx · Computer and output circuits / P068D

P068DCylinder 7 Glow Plug Control Circuit High

P068D indicates that the engine control module (ECM) has detected a high-voltage condition on the control circuit for cylinder 7's glow plug, which is used in diesel engines to aid cold starting. The ECM monitors the circuit and sets this code when the voltage on the control wire exceeds the expected range, typically indicating a short to power or an open circuit in the ground path.

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

Symptoms you may notice

  • Illumination of the check engine light (malfunction indicator lamp).
  • Hard starting or rough idle when the engine is cold, especially in low temperatures.
  • Excessive white smoke from the exhaust after a cold start due to incomplete combustion.
  • Engine misfire or rough running during the warm-up period.
  • Often no symptoms besides the warning light, as glow plug operation is only critical in cold weather.

Common causes, most likely first

  1. Faulty glow plug on cylinder 7 with an internal short to power.
  2. Damaged or chafed wiring in the glow plug control circuit, causing a short to voltage.
  3. Corroded or loose electrical connector at the glow plug or glow plug control module.
  4. Failed glow plug control module (also known as glow plug relay or preheat control unit) sticking closed.
  5. Open circuit in the ground path for the glow plug, causing the control circuit to see high voltage.
  6. Engine control module (ECM) internal fault, though this is rare.

How to diagnose and fix it

  1. Perform a visual inspection of the cylinder 7 glow plug, its connector, and the wiring harness for damage, corrosion, or loose connections.
  2. Use a digital multimeter to check the resistance of the glow plug; compare to specifications (typically 0.5 to 2 ohms) and replace if out of range.
  3. With the ignition off, disconnect the glow plug control module and check for battery voltage on the control circuit; if voltage is present, trace and repair the short to power.
  4. Check the ground circuit for the glow plug using a multimeter to measure resistance to chassis ground; repair any open or high-resistance ground.
  5. Test the glow plug control module by applying power and ground to its control terminals and verifying it switches voltage to the glow plug; replace if faulty.
  6. If all external circuits and components test good, suspect the ECM and perform further diagnostics or reprogramming as needed.

Glow Plug Circuit High Voltage Traps

  • A shorted glow plug can cause the control module to set a high-voltage code even though the plug itself is the root cause, so always test the plug before replacing the module.
  • Corrosion inside the glow plug connector often creates resistance that mimics a high-voltage fault, and cleaning or replacing the pigtail may solve the issue without any parts replacement.
  • The glow plug control module may be shared with other cylinders, so a fault in one circuit can sometimes affect others; check for related codes on other cylinders.
  • On some vehicles, the glow plug circuit is monitored only during the initial key-on preheat cycle, so the code may not set if the engine is already warm when scanned.
  • A high-voltage code can be set by a poor ground connection at the engine block, which raises the reference voltage on the control circuit; always verify engine grounds.

Can I keep driving with P068D?

You can generally drive the vehicle with this code, but cold starting may be difficult and the engine may run rough until warm; avoid heavy loads or long idling until repaired, and address it before winter weather arrives.

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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