P0679Cylinder 9 Glow Plug Circuit
An electrical fault has been detected in the glow plug circuit for cylinder 9, applicable to V10 and V12 diesel engines.
Definition
On large diesel engines with ten or more cylinders, the glow plug control module monitors each glow plug circuit for correct voltage and current during pre-glow and after-glow operation. P0679 sets when the cylinder 9 circuit shows an open, short, or abnormal current draw. Expect hard cold starting, white exhaust smoke, and rough idle until the engine warms, along with an illuminated MIL; a single failed plug may barely be noticeable in mild weather. The fault can also cause a cold-start emissions increase.
Common causes
- Failed cylinder 9 glow plug (open element)
- Corroded or loose connector at the glow plug terminal
- Damaged harness section (open or short to ground) serving cylinder 9
- Failed glow plug control module output stage
- Poor ground path raising circuit resistance
Common misdiagnoses
- Replacing the complete glow plug set when one circuit is faulty
- Replacing the glow plug control module before testing the plug
- Battery or starter work for a hard-cold-start complaint
- PCM replacement
Troubleshooting steps
1.Scan all modules for glow plug codes
A lone cylinder 9 code isolates the fault to that plug, its wiring, or one module driver.
2.Ohm-test the cylinder 9 glow plug
Disconnect the plug and measure element resistance to ground; compare with a known-good cylinder — an open reading confirms plug failure.
3.Inspect the circuit wiring
Trace the cylinder 9 feed wire for chafe points, heat damage, and corroded terminals at both ends.
4.Verify commanded output
Activate the glow system with a scan tool and check for supply voltage at the cylinder 9 terminal during the glow period.
5.Confirm on a cold restart
Clear the code and recheck after a full cold soak, since the diagnostic only runs when a glow cycle is commanded.