Failed glow plug on cylinder 9 (open element); Damaged, corroded or loose wiring or connector at the cylinder 9 glow plug; Poor power or earth connection to the glow plug control module; Blown fuse or failed relay in the glow plug supply; Faulty glow plug control module or driver output for that channel; Wiring harness break between the control module and the plug, for example from chafing or heat damage.

P0679

Cylinder 9 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
79: FaultThe specific fault in that area

What it means

P0679 means the engine control unit has detected an open circuit in the glow plug circuit for cylinder 9. The control unit monitors the glow plug output or its feedback signal and has seen no current flow, or a voltage that points to a break in the circuit. Because it refers to cylinder 9, it only applies to larger diesel engines such as V10 or V12 layouts, and the cylinder numbering depends on the engine.

Symptoms

Hard starting or extended cranking in cold weather
White smoke on start-up until the engine warms
Rough or uneven running when cold
Glow plug or engine warning light on the dashboard
Increased diesel knock for the first minutes after starting

Possible causes

Failed glow plug on cylinder 9 (open element)likely
Damaged, corroded or loose wiring or connector at the cylinder 9 glow pluglikely
Poor power or earth connection to the glow plug control modulepossible
Blown fuse or failed relay in the glow plug supplypossible
Faulty glow plug control module or driver output for that channelpossible
Wiring harness break between the control module and the plug, for example from chafing or heat damagerare

How to diagnose

Read all stored codes and freeze frame data, noting engine temperature and battery voltage when the fault set, and check for other glow plug codes.
Visually inspect the glow plug wiring, connector and harness near cylinder 9 for heat damage, chafing, corrosion or looseness.
Check the relevant fuses and relays and the battery and charging voltage, using the manufacturer's wiring diagram for your engine.
Check the glow plug control module power and earth connections for good contact and voltage drop.
Measure the cylinder 9 glow plug resistance and compare it with the specification for that engine; an open reading means the plug has failed.
With the connector unplugged, check the wiring for continuity and shorts between the plug and the control module.
Use a scan tool's live data or output test, if supported, to check that the control module drives the cylinder 9 channel.
If the plug and wiring are good, test the control module output as per the manufacturer's procedure.

How to fix

Replace the failed cylinder 9 glow plug, following the correct torque and removal procedure for your engine, as seized plugs can break.
Repair or replace damaged wiring and connectors, cleaning terminals as needed.
Replace the blown fuse or faulty relay after finding out why it failed.
Restore good power and earth connections to the glow plug module.
Replace the glow plug control module if its output is confirmed faulty, and carry out any coding or adaption the manufacturer requires.

Can I keep driving?

Low severity

The car can usually be driven, as the other cylinders still work, but cold starting may be poor and emissions higher until it is repaired. Book it in soon, especially before cold weather, and replace other plugs if they are of similar age.

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