P0678Cylinder 8 Glow Plug Circuit
An electrical fault has been detected in the glow plug circuit for cylinder 8 on a diesel engine.
Definition
The glow plug control module monitors voltage and current on each individual glow plug circuit while the plugs are energized before and after a cold start. P0678 sets when the cylinder 8 circuit is open, shorted, or drawing current outside the calibrated range. The engine may crank long and start hard in cold temperatures, run rough with white smoke until warm, and turn on the MIL; starting in warm conditions is usually normal. Repeated cold-start misfire increases emissions and fuel dilution of the oil.
Common causes
- Failed cylinder 8 glow plug (open heating element)
- Corroded or damaged glow plug connector terminal
- Open or shorted wiring between the glow plug control module and cylinder 8
- Failed output driver in the glow plug control module
- Poor engine or module ground
- Wrong-specification replacement glow plug
Common misdiagnoses
- Replacing all glow plugs for a single-circuit fault
- Replacing the glow plug relay/control module when the plug element is simply open
- Battery replacement for a cold-start complaint
- PCM replacement
Troubleshooting steps
1.Check for companion codes
Codes on several cylinders at once indicate a module, fuse, or ground problem rather than one bad plug.
2.Measure cylinder 8 glow plug resistance
With the terminal disconnected, a healthy plug reads roughly 0.5-6 ohms to ground; an infinite reading means the element is open.
3.Inspect wiring and the connector
Check for heat-damaged insulation, chafing, and green corrosion on the terminal at the plug.
4.Confirm output during a commanded glow cycle
Use a scan tool to activate the glow plugs and verify voltage arrives at the cylinder 8 terminal.
5.Isolate the module driver
Good plug and wiring with no output on the cylinder 8 pin at the module connector confirms a failed module driver.