Cylinder 8 Glow Plug Circuit Malfunction
The P0678 code indicates a fault in the glow plug circuit for cylinder 8 in a diesel engine. Glow plugs are used to preheat the combustion chamber for cold starts. The Powertrain Control Module (PCM) monitors the resistance of the glow plug circuit; when it detects an open or short circuit, it sets this code. This can be caused by a failed glow plug, damaged wiring, or a faulty glow plug control module.
01. Most Common Causes
- Faulty glow plug in cylinder 8
- Open or shorted wiring in the glow plug circuit
- Corroded or loose connector at the glow plug
- Failed glow plug control module or relay
02. Symptoms You Will Notice
- Hard starting, especially in cold weather
- Check Engine Light illuminated
- Rough idle or misfire on cold start
- Excessive white smoke from exhaust on startup
03. 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: Measure resistance of the glow plug; should be low (typically 0.5-2 ohms). Replace if open or shorted.
Step 4: Check voltage at the glow plug connector with key on; should be battery voltage. If not, test the glow plug relay and control module.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
You can drive short distances, but starting will be difficult in cold weather. Prolonged driving may cause unburned fuel to damage the catalytic converter.
How much does it cost to fix?▼
Replacing a single glow plug typically costs $50-$150 including parts and labor. If the control module or wiring needs repair, costs can be higher.