Cylinder 8 Glow Plug Circuit Malfunction
The P0678 code indicates a fault in the glow plug circuit for cylinder 8. Glow plugs are used in diesel engines to preheat the combustion chamber for cold starts. This code is triggered when the powertrain control module (PCM) detects an abnormal voltage or resistance in the circuit, often due to a faulty glow plug, open or shorted wiring, or a failed glow plug relay. This can lead to hard starting, misfires, and increased emissions.
1. Most Common Causes
- Open or internally shorted glow plug in cylinder 8
- Damaged, corroded, or disconnected wiring or connector in the cylinder 8 glow plug circuit
- Short to ground in the glow plug harness
- Short to voltage in the glow plug control circuit
- Faulty glow plug relay (if applicable)
- Failed glow plug control module
- Internal ECM failure (less common)
2. Symptoms You Will Notice
- Hard starting, especially in cold weather
- Engine misfire or rough idle
- Check Engine Light illuminated
- Increased exhaust smoke on startup
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 8 for damage or corrosion.
Step 3: Test the glow plug resistance with a multimeter (should be low, typically 0.5-2 ohms).
Step 4: Check voltage at the glow plug connector with the ignition on (should be battery voltage).
Step 5: If wiring and voltage are okay, replace the glow plug for cylinder 8.
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, battery drain, and potential damage to the starter. 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. Costs may vary if wiring or relay repairs are needed.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.