What it means
The glow plug circuit for cylinder 9 has a detected fault. The code does not by itself identify whether the cause is the plug, wiring, or control module.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The glow plug control module supplies the glow plug for cylinder 9 to help the cylinder ignite fuel during cold operation. P0679 indicates that the cylinder 9 glow plug circuit has an electrical fault, but the code alone does not distinguish a failed plug from wiring, connector, or module-output trouble. Cylinder 9 identification and circuit layout should be confirmed in the vehicle’s service information.
Common causes
Most likely first.
- Failed glow plug
- Open or shorted wiring or poor connector contact
- Glow plug control module fault
Symptoms
- Glow plug warning light on (often orange coil symbol)
- Hard cold start or no start in cold weather
- White smoke from exhaust during warm-up
How to diagnose it
- Save freeze-frame data and check for other glow-plug codes to see whether the fault appears isolated to cylinder 9 or affects multiple circuits.
- Use service information to locate cylinder 9, then inspect its glow plug connector and lead for corrosion, looseness, chafing, or heat damage.
- With the circuit isolated as directed by the manufacturer, test the cylinder 9 glow plug and compare the result with the manufacturer’s specification.
- Check the cylinder 9 wiring for opens, shorts, and poor terminal contact between the plug and module.
- If the plug and wiring pass, test the glow plug module’s cylinder 9 output using the manufacturer’s procedure.
- Clear the code after repair and confirm it does not return during a cold-start cycle.
Common fixes
- Clean or secure a poor cylinder 9 connector contact.
- Repair damaged wiring in the cylinder 9 glow plug branch.
- Replace the cylinder 9 glow plug if it fails the specified test.
- Repair or replace the module’s cylinder 9 output if the plug and wiring test good.
Can I keep driving?
If the engine runs smoothly once warm, limited driving may be possible, but cold starting can become difficult and warm-up may bring rough running or white smoke. Avoid repeated cranking and address the fault promptly if starting or running worsens.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.