What Does P0679 Mean?
The PCM has detected a malfunction in the cylinder 9 glow plug circuit on a diesel engine. This is a rare code that applies only to engines with 9 or more cylinders, such as certain large V10 or V12 diesel engines found in heavy-duty commercial vehicles, marine applications, or some European luxury vehicles. The cylinder 9 glow plug is not operating within expected parameters, affecting cold-start combustion in that cylinder. On engines with this many cylinders, the impact of a single glow plug failure is relatively minor.
Common Causes
40%
Failed cylinder 9 glow plug (open or degraded element)
25%
Corroded or loose bus bar connection at cylinder 9
20%
Wiring fault in the cylinder 9 glow plug circuit
10%
Glow plug control module output failure for cylinder 9
5%
Glow plug seized in cylinder head due to high mileage
Diagnostic Steps
1
Identify the cylinder 9 location on the specific engine configuration — on V10 engines (e.g., VW/Audi 5.0 V10 TDI), cylinder numbering may differ between manufacturers. Consult the service manual.
2
Disconnect the cylinder 9 glow plug and measure resistance — spec is typically 0.5-2.0 ohms. On some large-displacement diesels, the spec may be lower (0.3-0.8 ohms).
3
Check voltage at the glow plug connector during the glow cycle. On multi-cylinder engines, verify the glow plug module is individually addressing cylinder 9 and not grouping plugs.
4
Inspect the bus bar or individual wiring for cylinder 9 — on large engines, harness routing can be complex and the cylinder 9 connection may be in a difficult-to-access location.
5
If multiple glow plug codes are present across the engine, consider replacing all glow plugs as a set. On high-cylinder-count engines, this is a significant job and best done as preventive maintenance.
Estimated Repair Cost
$100 - $700
Parts + labor, varies by vehicle and location
The PCM has detected a malfunction in the cylinder 9 glow plug circuit on a diesel engine. This is a rare code that applies only to engines with 9 or more cylinders, such as certain large V10 or V12 diesel engines found in heavy-duty commercial vehicles, marine applications, or some European luxury ...
The most common cause of P0679 (Cylinder 9 Glow Plug Circuit) is: Failed cylinder 9 glow plug (open or degraded element)
Typical repair costs for P0679 range from $100 to $700, depending on the vehicle, location, and whether you do it yourself or go to a shop.
On engines with 9+ cylinders, a single failed glow plug has minimal impact on cold starting. The remaining glow plugs provide sufficient heating for reliable cold starts in most conditions. However, if this code appears with multiple other glow plug codes, the cumulative effect could prevent cold starting. These high-cylinder-count engines are expensive to service, so address glow plug issues before they cascade.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0679 to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Glow Plug System
Difficulty
Type
Generic (SAE)
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