P0679Powertrain
Powertrain · SAE
Glow Plug Cylinder 9 — Circuit
Meaning
The control module has detected an electrical fault in the glow plug circuit for cylinder 9. The fault may be an open or shorted glow plug, or a problem in the wiring or glow plug control module.
Common causes
- Cylinder 9 glow plug open or shorted to ground
- Corroded or broken connector or wiring to the glow plug
- Defective glow plug control module (GSC) or relay
- Glow plug failed due to prolonged overheating or high mileage
- Missing supply voltage or ground in the control circuit
Symptoms
- Check engine light on
- Hard cold start, especially on high-cylinder-count engines
- Rough combustion and shudder at cylinder 9 during warm-up
- Whitish smoke on start-up
- Possible pre-glow indicator light (model-specific)
Severity
Moderate: the vehicle remains drivable, but cold starting is impaired; prolonged operation can cause carbon deposits in the cylinder and cylinder head damage.
What to do
- Measure glow plug resistance at cylinder 9 (typical 0.5–4 Ω); replace if abnormal
- Check connector and wiring for oxidation, corrosion, or physical breaks
- Verify supply voltage at the GSC module and ground integrity
- Perform a functional test of the glow plug control module with a scan tool
- Replace the GSC module if wiring is intact but the plug is not being commanded