Cylinder 9 Glow Plug Circuit
The glow plug for cylinder 9 isn't heating up properly before a cold diesel start, like a lighter that won't ignite. The engine's computer detected a circuit problem preventing the glow plug from receiving or maintaining the correct electrical signal.
The ECM monitors voltage and current draw through the cylinder 9 glow plug circuit during the glow plug preheat cycle. It detects open circuits, shorts to ground, or excessive resistance that prevents proper heating. The control module expects a specific voltage drop and amperage within defined timing windows.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Glow Plug Circuit Voltage | 11-14V during preheat cycle | Below 8V or no voltage detected |
| Circuit Resistance | 0.5-2 ohms per plug | Above 10 ohms or infinite (open circuit) |
Code P0679 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0679 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.