P0679
UniversalThe Powertrain Control Module (PCM) has detected a malfunction in the electrical circuit of the glow plug for cylinder 9.
Medium SeverityThis code indicates a problem with the electrical circuit for the glow plug in cylinder 9. This usually means the glow plug itself is faulty, or there's an issue with its wiring or control, which can lead to difficulty starting the engine, especially in cold weather.
⚠️ Before You Begin (Safety First)
Always allow the engine to cool completely before working on any engine components. Disconnect the negative battery terminal to prevent accidental electrical shorts or injury. Use proper eye protection and gloves.
Common Symptoms
- Hard starting in cold weather
- Check Engine Light illumination
- Rough idling for a short period after starting
- Increased exhaust smoke (white or blue) during cold starts
- Reduced fuel economy (less common but possible)
Common Causes
- Faulty glow plug (most common)
- Damaged or corroded wiring to the glow plug
- Faulty glow plug control module or relay
- Open or short circuit in the glow plug wiring
- Carbon buildup around the glow plug tip (making removal difficult)
Frequently reported on: Ford F-Series Super Duty (Diesel models), Ram Heavy Duty (Diesel models), Chevrolet Silverado/GMC Sierra HD (Diesel models), Volkswagen Touareg V10 TDI (2004-2008)
Diagnostic Steps
- Visually inspect the glow plug and its wiring for signs of damage or corrosion.
- Test the resistance of the glow plug using a multimeter. Compare to manufacturer specifications (usually very low ohms, e.g., 0.6-2 ohms).
- Check for voltage at the glow plug connector during cold start (briefly, typically 5-10 seconds) to verify the control module is sending power.
- If the glow plug is faulty, carefully remove and replace it. Ensure the engine is cold to avoid damaging the cylinder head threads.
- Clear the fault code using an OBD-II scanner and test the starting performance.
👨🔧 When to Call a Professional
If you have tested the glow plug and its direct wiring, and the issue persists, or if the glow plug is seized and cannot be removed without specialized tools, it's best to consult a professional technician. They can diagnose deeper electrical issues, the glow plug control module, or safely extract a broken glow plug.
Commonly Asked Questions
Can I drive with a P0679 code?
Yes, you can typically drive with a P0679 code, but starting in cold weather will be difficult, and the engine may run rough until it warms up. It's best to address it promptly to prevent further issues, such as increased emissions or stress on the starting system.
Is P0679 an expensive repair?
Replacing a single glow plug is relatively inexpensive, with the part itself often costing between $20-$50. However, labor costs can vary significantly based on engine accessibility and if the plug is seized or breaks during removal.
How do I identify Cylinder 9?
Refer to your vehicle's service manual or a reliable repair guide for the exact cylinder numbering scheme. It can vary by manufacturer and engine type, especially on larger or unique engine configurations.
What tools do I need to replace a glow plug?
You'll typically need a socket wrench set (often a deep socket for glow plugs), a torque wrench, a multimeter for testing, and possibly penetrating oil for seized plugs. A specialized glow plug removal tool might be needed if the plug is stuck.
Can a bad glow plug affect fuel economy?
While not the primary symptom, consistent hard starts, extended warm-up times, and incomplete combustion in cold conditions due to a faulty glow plug can subtly impact fuel efficiency and increase exhaust emissions over time.