What Does P0678 Mean?
The PCM has detected a malfunction in the cylinder 8 glow plug circuit on a diesel V8 engine. The glow plug or its wiring is not functioning within expected electrical parameters. Cylinder 8 will not receive proper pre-heating for cold combustion, causing that cylinder to misfire during cold starts until the engine warms up. The driver may notice slightly harder cold starting and a brief rough idle during warm-up, though on a V8 the impact of a single glow plug failure is less severe.
Common Causes
40%
Failed cylinder 8 glow plug
25%
Corroded or loose connection at cylinder 8 terminal
20%
Wiring fault in cylinder 8 glow plug circuit
10%
Glow plug control module output failure for cylinder 8
5%
Glow plug seized in cylinder head requiring extraction
Diagnostic Steps
1
Disconnect the cylinder 8 glow plug and measure resistance — expected 0.5-2.0 ohms. Compare to all other cylinders to identify degradation patterns.
2
Verify voltage at the cylinder 8 bus bar connection during the glow cycle. No voltage means the issue is in the supply circuit, not the glow plug.
3
Cylinder 8 on V8 diesels is typically the rearmost cylinder on bank 2. Check access requirements — on many applications, the intake manifold or turbo plumbing may need to be moved for access.
4
Inspect the entire bank 2 bus bar for damage, loose connections, or corrosion. Cylinder 8 is often the end of the bus bar chain and most susceptible to connection issues.
5
When all glow plugs are tested, if more than 2 are out of spec, replace the complete set. Glow plugs on V8 diesels should be treated as a maintenance item every 100,000 miles.
Estimated Repair Cost
$80 - $600
Parts + labor, varies by vehicle and location
The PCM has detected a malfunction in the cylinder 8 glow plug circuit on a diesel V8 engine. The glow plug or its wiring is not functioning within expected electrical parameters. Cylinder 8 will not receive proper pre-heating for cold combustion, causing that cylinder to misfire during cold starts ...
The most common cause of P0678 (Cylinder 8 Glow Plug Circuit) is: Failed cylinder 8 glow plug
Typical repair costs for P0678 range from $80 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Not an immediate safety issue. A single failed glow plug on a V8 diesel has minimal impact on cold starting in moderate temperatures. In extreme cold or when combined with other failed plugs, starting becomes increasingly difficult. Diesel V8 trucks are often used in demanding conditions where reliable starting is critical.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0678 to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Glow Plug System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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