Cylinder 9 Glow Plug Circuit Malfunction
The P0679 code indicates a fault in the glow plug circuit for cylinder 9. This circuit includes the glow plug itself, wiring, connectors, and the glow plug control module. A malfunction can prevent the cylinder from heating properly during cold starts, leading to hard starting, increased emissions, and rough idle until the engine warms up.
1. Most Common Causes
- Faulty or burned-out glow plug in cylinder 9
- Damaged, corroded, or disconnected wiring or connector in the glow plug circuit
- Short to ground in the glow plug circuit wiring
- Short to voltage in the glow plug circuit wiring
- Open circuit in the glow plug circuit wiring
- Defective glow plug relay
- Failed glow plug control module (if applicable)
- Internal ECM failure (rare)
2. Symptoms You Will Notice
- Hard starting, especially in cold weather
- Rough idle or misfire on cylinder 9 until engine warms up
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for other related codes.
Step 2: Visually inspect the glow plug wiring and connector for cylinder 9 for damage or corrosion.
Step 3: Measure the resistance of the glow plug for cylinder 9; compare to specifications (typically 0.5-2 ohms).
Step 4: If the glow plug is good, test the circuit voltage at the connector with the key on; should see battery voltage.
Step 5: If voltage is present, replace the glow plug. If not, check the control module and wiring.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
You can drive short distances in mild weather, but cold starts will be difficult and may cause excessive cranking and battery drain. It's best to repair promptly.
How much does it cost to fix?▼
Typically $50-$150 for a single glow plug replacement, including parts and labor. If the control module or wiring needs repair, costs can be higher.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.