Failed glow plug on cylinder 8 (open-circuit heating element); Damaged, corroded or loose wiring or connector between the glow plug and the glow plug control module; Corroded or loose glow plug terminal or connector nut at the plug; Blown fuse or fusible link in the glow plug supply circuit; Faulty glow plug control module or driver channel for cylinder 8; Fault in the engine control unit.

P0678

Cylinder 8 Glow Plug Circuit/Open.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
78: FaultThe specific fault in that area

What it means

P0678 means the engine control unit has detected an open circuit in the glow plug circuit for cylinder 8. The control unit monitors the glow plug feed or driver circuit and found no current flow or an out-of-range voltage when it expected the glow plug to be energised. This only applies to diesel engines with at least eight cylinders.

Symptoms

Hard starting or extended cranking when the engine is cold
White smoke on cold start-up
Rough running or misfire on cylinder 8 until the engine warms up
Glow plug warning light on or flashing, or engine management light on
Increased exhaust emissions or a diesel knock when cold

Possible causes

Failed glow plug on cylinder 8 (open-circuit heating element)likely
Damaged, corroded or loose wiring or connector between the glow plug and the glow plug control modulelikely
Corroded or loose glow plug terminal or connector nut at the plugpossible
Blown fuse or fusible link in the glow plug supply circuitpossible
Faulty glow plug control module or driver channel for cylinder 8possible
Fault in the engine control unitrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting engine temperature and battery voltage when the fault set
Check for other glow plug codes, as several codes point towards a shared supply, fuse or control module fault
Visually inspect the cylinder 8 glow plug wiring, connector and terminal for chafing, heat damage, corrosion or looseness
Check the glow plug fuse or fusible link and the supply voltage to the control module
Measure the cylinder 8 glow plug resistance with the wiring disconnected and compare it with the manufacturer specification
With the circuit powered up, check for supply voltage and a good earth at the glow plug connector as the procedure for your engine specifies
Check the wiring between the control module and the glow plug for continuity and short circuits, following the wiring diagram for your model
Use a bidirectional test to command the glow plugs on, if available, and check the module output for cylinder 8

How to fix

Replace the faulty cylinder 8 glow plug, using the correct torque and taking care not to snap it in the cylinder head
Repair or replace damaged wiring, terminals or connectors in the cylinder 8 circuit
Clean and refit the glow plug connection, replacing any corroded terminal parts
Replace the blown fuse or fusible link after finding out why it failed
Replace the glow plug control module if its output for cylinder 8 is proven faulty, and have it coded to the vehicle if the model requires it

Can I keep driving?

Low severity

The car can normally be driven, but cold starts may be poor and emissions higher until it is repaired. Book it in soon, especially before cold weather, and avoid repeated long cranking attempts.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.