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DTC codes / P06xx · Computer and output circuits / P0679

P0679Cylinder 9 Glow Plug Circuit

P0679 indicates that the engine control module has detected an open circuit, short to ground, or short to power on the glow plug circuit for cylinder 9 of a diesel engine. The module monitors the glow plug control circuit and sets this code when the expected current flow or voltage feedback does not match the commanded state during the glow plug preheat or afterglow phase.

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

Symptoms you may notice

  • Illuminated check engine light or glow plug warning lamp
  • Hard cold starting or extended cranking before the engine fires
  • Rough idle and white smoke immediately after a cold start
  • Reduced engine performance or limp mode in some vehicles
  • Often no symptoms besides the warning light, especially in warm weather

Common causes, most likely first

  1. Failed glow plug on cylinder 9 (open heating element)
  2. Damaged or corroded electrical connector at the cylinder 9 glow plug
  3. Chafed, broken, or shorted wiring in the glow plug harness near cylinder 9
  4. Faulty glow plug control module or relay affecting that circuit
  5. Loose or corroded ground connection for the glow plug circuit
  6. Engine control module internal driver fault for the cylinder 9 glow plug output

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data to confirm the fault is current and specific to cylinder 9
  2. Visually inspect the cylinder 9 glow plug connector and harness for corrosion, chafing, or loose pins
  3. Measure glow plug resistance with a digital multimeter and compare to the manufacturer's specification
  4. Check for voltage drop and continuity on the glow plug feed and ground circuits while cranking or during glow plug activation
  5. Test the glow plug control module output with a bidirectional scan tool or oscilloscope if the circuit tests good
  6. Replace the faulty glow plug, connector, or harness section as indicated by test results
  7. Clear the code and perform a cold-start drive cycle to verify the repair and confirm the code does not return

Glow plug circuit checks that save misdiagnosis

  • A glow plug that tests good on resistance can still set P0679 if the control module does not see the expected current ramp, so circuit current testing with a clamp or scope is more reliable than a simple ohm check.
  • Corrosion inside the glow plug connector often mimics a failed glow plug, and replacing the plug without repairing the connector leads to a repeat code within days.
  • On engines with a shared glow plug control module, a single failed output driver can set a cylinder-specific code like P0679 even though all glow plugs are healthy.
  • P0679 does not always mean the glow plug is dead; a short to ground on the control wire can command the module off and set the same code, so inspect the harness routing near hot or moving components.
  • Cold weather makes the fault more noticeable, but the code can set in warm weather during the afterglow phase, so a lack of cold-start symptoms does not rule out a circuit problem.

Can I keep driving with P0679?

You can usually drive the vehicle, but cold starting may be difficult and prolonged cranking can stress the starter and battery. Avoid repeated long cranking attempts and have the circuit diagnosed before cold weather arrives, especially if the engine runs rough or smokes after start.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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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