P0678
UniversalThe Powertrain Control Module (PCM) has detected an open circuit, short circuit, or out-of-range resistance in the glow plug circuit for cylinder 8.
Medium SeverityThis code indicates an electrical problem with the glow plug for cylinder 8, which is crucial for starting a diesel engine in cold weather. While it might not immediately affect driving, it can lead to difficult or no-start conditions, especially in colder temperatures.
⚠️ Before You Begin (Safety First)
Always allow the engine to cool completely before working on glow plugs to avoid burns. Disconnect the battery before attempting any electrical repairs to prevent accidental shorts. Be cautious when removing glow plugs as they can seize and break inside the cylinder head.
Common Symptoms
- Check Engine Light (CEL) illuminated
- Difficulty starting the engine, especially in cold weather
- Rough idling or misfires shortly after cold start
- Increased exhaust smoke during cold starts
Common Causes
- Faulty or burned out glow plug for cylinder 8
- Wiring harness open or shorted in the glow plug circuit
- Corrosion or poor connection at the glow plug connector
- Faulty glow plug control module/relay
- Damaged Powertrain Control Module (rare)
Frequently reported on: Ford F-Series (Diesel), Ram 2500/3500 (Cummins), GM Silverado/Sierra 2500/3500 (Duramax), Volkswagen TDI Models, Mercedes-Benz Sprinter
Diagnostic Steps
- Gather tools: multimeter, socket set (including deep sockets for glow plugs), torque wrench, penetrating oil.
- Locate cylinder 8 glow plug. Consult your vehicle's service manual for exact location.
- Disconnect the battery. Carefully remove the electrical connector from the glow plug.
- Test the resistance of the glow plug using a multimeter. Compare to manufacturer specifications. An open circuit or very high resistance indicates a faulty plug.
- Apply penetrating oil around the base of the glow plug and allow it to soak.
- Carefully attempt to remove the glow plug. If it resists, apply more penetrating oil and wait, or gently rock it to loosen. Be extremely careful to avoid breaking it.
- Install the new glow plug, ensuring it's tightened to the manufacturer's specified torque to prevent damage to the cylinder head.
- Reconnect the electrical connector and the battery. Clear the code and test the engine.
👨🔧 When to Call a Professional
If you attempt to remove the glow plug and it feels seized, or if you lack the specialized tools to extract a broken glow plug, it's best to consult a professional mechanic to prevent costly damage to the cylinder head.
Commonly Asked Questions
What is a glow plug and why is it important?
A glow plug is an electrical heating device used to aid in starting diesel engines. It heats the combustion chamber to a high temperature, helping to ignite the fuel in cold weather conditions.
Can I drive with a P0678 code?
You can typically drive with a P0678 code, but you may experience difficult starting, especially in cold weather, and potentially rough idling until the engine warms up. It's best to address it promptly to prevent further issues.
Is it difficult to replace a glow plug?
Replacing a glow plug can range from medium to advanced difficulty. Access can be challenging, and glow plugs can seize in the cylinder head, making removal risky. Proper tools and technique are essential.
What tools do I need to replace a glow plug?
You will typically need a socket set, a deep socket specifically for glow plugs, a torque wrench, a multimeter for testing, and penetrating oil. Some jobs may require specialized glow plug removal tools.
Could a P0678 code indicate a larger problem?
While often just a faulty glow plug, sometimes the issue could stem from the glow plug control module or even wiring damage. Diagnosing the exact cause with a multimeter is crucial.