Cylinder 8 Glow Plug Circuit
The P0678 code means the vehicle's onboard diagnostic system has detected: Cylinder 8 Glow Plug Circuit. This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- moderate
- Estimated cost
- $50 - $500 (parts and labor)
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Common Symptoms
- Check Engine Light on
- Cylinder 8 glow plug not heating
- Reduced system performance
- Warning indicators on dashboard
Probable Causes (Ranked by Likelihood)
- Failed glow plug in Cylinder 8 Very High
- Wiring damage or corrosion in the circuit High
- Poor electrical connection at module or component Moderate
- ECM or control module internal failure Low
Step-by-Step Diagnostic Procedure
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Use a scan tool to read data related to Cylinder 8 Glow Plug
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Inspect all related wiring, connectors, and grounds for damage, corrosion, or loose connections
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Measure circuit voltage, resistance, and continuity to identify the fault type
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Check for technical service bulletins related to this code for your specific vehicle
Common Fixes by Vehicle
What techs usually find when diagnosing P0678 on specific vehicles — tap your vehicle for the fix and the exact part:
2001-2010 Duramax 6.6L hard DIY
P0678 = Glow Plug #8 (passenger-side rear). This is the hardest plug to reach on Duramax -- the turbocharger downpipe is in the way on some years. Replace all 8 while you're in there. AC Delco OE or NGK Y-505J plugs, 11 ft-lb torque.
Labor: 3-5 hours2003-2007 Ford 6.0L Power Stroke hard DIY
6.0L P0678 = glow plug #8. The driver's rear on Ford is tight against the firewall. Use a ratcheting 1/4" drive with a swivel and patience. Replace all 8 with Motorcraft ZD-11.
Labor: 4-6 hours2008-2010 Ford 6.4L Power Stroke hard DIY
6.4L P0678 = glow plug #8. Cab-off service makes this dramatically easier. Motorcraft ZD-13. Check the EGR cooler and oil cooler while the cab is off -- these are common 6.4L failure points and costlier to do later.
Labor: 6-10 hoursFrequently Asked Questions
What happens when Cylinder 8 glow plug fails?
A single failed glow plug in Cylinder 8 may cause hard starting in cold weather and rough running until the engine warms up. In mild temperatures, you may not notice any symptoms. Multiple failed glow plugs cause progressively worse cold starting.
How much does a glow plug replacement cost?
Individual glow plugs cost $15 to $50 each. Labor is the main expense as some glow plugs are difficult to access. A single glow plug replacement typically costs $100 to $300. Many shops recommend replacing all glow plugs at once.
Can a glow plug break off during removal?
Yes, especially on high-mileage diesel engines. Carbon buildup and corrosion can seize the glow plug in the cylinder head. Special extraction tools exist for broken glow plugs. Applying penetrating oil days before removal helps.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0678 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
Explore More
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