OBD-II trouble code
P0679: Cylinder 9 Glow Plug Circuit
A fault on the cylinder 9 glow plug channel. Very few road vehicles have nine cylinders, so the first job on this code is not diagnosis at all — it is confirming the code is genuine, because a P0679 on an engine that has fewer than nine cylinders is telling you about your scan tool, not your engine.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $1,800
- DIY difficulty
- Advanced DIY
What does P0679 mean?
P0679 is defined as a fault on the cylinder 9 glow plug circuit. Taken at face value it is another branch code: element, terminal, wire, controller output. But there is a step that comes before all of that on this particular code, and skipping it is how people spend an afternoon chasing a cylinder their engine does not have.
**Count the cylinders.** A ninth cylinder in a road-going diesel means a V10 or a V12, and those are rare, low-volume machines. Outside passenger vehicles the number is more plausible — industrial, agricultural, marine and generator engines routinely run nine or more cylinders, and some of them report through OBD-style diagnostics. But if the engine in front of you is a four, a five, a six or an eight, then the code you are reading is not describing a real circuit on it, and every hour spent testing is wasted.
There are a few ordinary reasons a phantom P0679 appears, and none of them requires anything to be wrong with the glow plug system:
**The reader is guessing.** Inexpensive code readers carry a generic lookup table. When a module reports a manufacturer-specific code the table does not contain, some tools display the nearest generic definition rather than admitting they do not know. The number on the screen is real; the description under it was invented by the tool.
**The code came from a different module.** On a vehicle with several controllers it is easy to read a stored code from a module that is not the engine controller and attribute it to the engine. The same five characters can mean different things in different systems.
**The controller's software does not match the engine.** A remanufactured, donor or aftermarket module flashed with a calibration for a different engine can enumerate channels the vehicle does not physically have, and then report faults on them. This also shows up after a module swap that was never properly programmed to the vehicle.
So the sequence is: confirm the cylinder count, then re-read the codes with a tool that carries the manufacturer's own definitions rather than a generic table, and check which module actually stored it. **If the code survives all three checks on a genuine V10 or V12, then diagnose it as a normal glow plug branch fault** — measure the plug's resistance against its neighbours, check the terminal and the feed under load, and work outward to the harness and the controller output.
One thing worth knowing before you price the parts on such an engine: **large, high-output diesels of this class frequently do not use the inexpensive metal-sheathed glow plug that the four-cylinder world runs on.** Ceramic and other high-performance plug types are common on them, and they cost a multiple of the ordinary part. Any budget figure carried over from a small-diesel article will be badly wrong here, so price the actual part number before agreeing to anything.
Common causes
- Generic code reader displaying an approximate definition for a manufacturer-specific code the tool does not recognise
- Code stored by a module other than the engine controller and misattributed
- Control module flashed with a calibration for a different engine, enumerating channels the vehicle does not have
- Cylinder 9 glow plug element open or drifted high, on a genuine V10 or V12
- Corroded or loose terminal at the cylinder 9 plug
- Damaged branch wire or bank harness section serving that cylinder
- Failed cylinder 9 output channel in the glow plug control module
- Shared supply, ground, relay or high-current fuse fault where multiple channels report
- Incorrect or mixed-specification plugs fitted during a previous repair
Symptoms
- A glow plug code for a cylinder the engine does not appear to have
- Codes that read differently on two different scan tools
- Extended cranking on a cold start, where the engine genuinely has nine or more cylinders
- Rough running and misfire during the first minute from cold
- White smoke on cold start that clears with temperature
- Check engine light with P0679 stored alongside other glow plug codes
- Recent control module replacement, reflash or engine swap preceding the code
Diagnostic steps
- 1.Confirm the engine's cylinder count and configuration from the VIN or engine code before doing anything else. If it has fewer than nine cylinders, stop diagnosing the glow plug system.
- 2.Re-read the codes with a scan tool that carries the manufacturer's own definitions rather than a generic lookup table, and compare the two readings.
- 3.Identify which module actually stored the code. A code read at vehicle level may not have come from the engine controller.
- 4.Check the service history for a module replacement, reflash, donor part or engine swap, any of which can leave a controller enumerating channels the engine does not have.
- 5.If the code is confirmed genuine on a V10 or V12, look up that engine's cylinder numbering and locate cylinder 9 physically before removing anything.
- 6.Measure the resistance of every glow plug and compare cylinder 9 against the rest. Matched readings with one outlier identify the failed element.
- 7.Measure voltage at the cylinder 9 terminal during an energised preheat cycle so a resistive connection is not mistaken for a dead plug.
- 8.Inspect the terminal, connector and branch wire for corrosion, heat damage and chafe.
- 9.Price the actual part number before ordering, since engines of this class often use ceramic or high-output plugs that cost far more than a standard metal-sheathed plug.
- 10.Verify the repair after a genuine overnight cold soak, because the glow circuit is only tested during preheat.
Repair cost
$0 – $1,800
If the code turns out to be a tool artefact or a module calibration mismatch, the correct spend on the glow plug system is nothing — re-reading with a proper scan tool costs a diagnostic fee at most. Diagnosis is $100 to $220 on a vehicle of this class. Where the fault is genuine, plugs for large high-output diesels are frequently ceramic or high-performance types at $60 to $200 each rather than the $25 part common on small diesels. A single replacement with workable access is $250 to $700 fitted; a poorly accessible cylinder on a V10 or V12 can run $700 to $1,500. Harness or terminal repair is $150 to $500. A control module is $400 to $1,200 fitted, and on some of these vehicles it requires dealer-level programming.
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DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.