The P0678 code is a generic OBD-II diagnostic trouble code indicating “Cylinder 8 Glow Plug Circuit.” It means the powertrain control module detected an electrical fault in the glow plug circuit for the eighth cylinder.
This code does not automatically prove the glow plug has failed. Diagnosis requires testing the circuit resistance, inspecting the wiring, and verifying the control module signals before replacing components.
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Key Takeaways
- The P0678 code indicates the control module detected an open or short circuit in the cylinder 8 glow plug control circuit.
- Possible causes include a failed glow plug, damaged wiring, corroded connectors, or a faulty glow plug control module.
- Diagnosis involves measuring the glow plug’s electrical resistance, testing the control circuit voltage, and inspecting the harness.
- Repairs may involve replacing the confirmed failed cylinder 8 glow plug, repairing wiring, or replacing the control module.
P0678 Code Description and Implications
The P0678 code indicates that the powertrain control module detected an electrical fault in the cylinder 8 glow plug circuit. This generic category of error codes applies to diesel engines equipped with glow plugs.
Glow plugs heat the combustion chamber to aid cold starting and reduce emissions. The control module monitors the electrical resistance and voltage feedback of each glow plug circuit to verify proper operation.
When the module detects an open circuit, short to ground, or short to voltage specifically on the cylinder 8 circuit, it stores this code and may disable the glow plug system.
Common Causes of the P0678 Code
A failed cylinder 8 glow plug is a primary cause. The internal heating element can break, creating an open circuit, or short out internally, triggering the electrical fault code.
Damaged wiring, chafed insulation, or corroded connectors in the harness between the control module and the cylinder 8 glow plug will interrupt the control signal or cause a short.
A faulty glow plug control module or relay can fail to send the proper voltage to the glow plugs, though this typically affects multiple cylinders rather than just cylinder 8.
Symptoms Associated With P0678
An illuminated Check Engine Light or glow plug warning light is expected once the monitor detects the circuit fault. MIL behavior varies by application, but the light commands when the fault is verified.
You may experience extended cranking times, hard starting, or white smoke from the exhaust during cold starts due to incomplete combustion in cylinder 8.
In some applications, the engine may idle roughly or misfire until the combustion chamber reaches operating temperature.
Diagnosing the P0678 Trouble Code
Diagnosis must begin by confirming the exact factory definition and checking for related glow plug codes. Record freeze-frame data to see the engine temperature and voltage states when the code set.
Perform a thorough visual inspection of the wiring harness and connectors at the cylinder 8 glow plug. Look for chafed wires, broken insulation, corrosion, or oil inside the connector.
Use a multimeter to test the cylinder 8 glow plug’s internal resistance and compare it to the exact manufacturer specification and the resistance of the other cylinders.
Test the control circuit for an open circuit, short to ground, or short to voltage. If the plug and wiring test correctly, the fault may lie within the control module.
Repairing the P0678 Issue
Repairs must follow confirmed diagnosis. If the wiring harness is found to be broken, shorted, or corroded, repair the circuit and secure the harness away from hot exhaust components.
If electrical testing confirms the cylinder 8 glow plug is internally open or shorted, replace the glow plug using a component meeting the exact vehicle specification.
If the manufacturer diagnostic procedure confirms the glow plug control module as the fault, follow the applicable module service or replacement procedure.
Cost Considerations for P0678 Repairs
Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.
Replacing a single glow plug on an easily accessible engine involves minimal parts and moderate labor expenses.
If the glow plug is seized in the cylinder head or requires extensive disassembly for access, the labor costs will increase significantly. Control module replacement also increases parts and programming costs.
Prevention Strategies for Future Occurrences
Inspecting exposed engine wiring harnesses and electrical connectors during routine under-hood service can help identify developing insulation damage or moisture intrusion before it causes a circuit fault.
Addressing engine oil or coolant leaks promptly helps prevent fluid from wicking into electrical connectors and disrupting the glow plug control circuits.
Some electrical glow plug failures occur due to normal component aging and harsh under-hood environments, and cannot be reliably prevented.
Troubleshooting Tips and Best Practices
Do not attempt to fix a P0678 code by simply replacing the glow plug without checking the wiring and connector first. Chafed wires near the valve cover are a frequent cause.
Always verify the exact physical location of cylinder 8. On many V8 diesel applications, cylinder 8 is located at the rear of the engine block, requiring specific access.
Check for technical service bulletins for your exact vehicle application, as some manufacturers release updated wiring harness pigtails or revised glow plug designs to address chronic circuit faults.
More OBD-II Codes
| P0677 | P0676 | P0675 | P0674 |
| P0673 | P0672 | P0683 | P0685 |
| P0688 | P0690 | P0691 | P0700 |
| P0670 | P0671 | P0602 | P0604 |
| P0605 | P0606 | P0607 | P0625 |
| P0620 | P0638 | P0641 | P0652 |
Frequently Asked Questions
Is the P0678 code generic or manufacturer-specific?
Yes, P0678 is a generic SAE OBD-II diagnostic trouble code. However, the exact glow plug circuit architecture and testing procedures vary by manufacturer and diesel engine application.
Will a P0678 code cause my diesel engine to stall?
It typically will not cause stalling once the engine is warm. However, you may experience extended cranking, hard starting, or rough idling during cold starts due to incomplete combustion in cylinder 8.
Does a P0678 code mean my cylinder 8 glow plug is broken?
Not necessarily. While the glow plug can fail, the code is also frequently caused by a shorted wire, a corroded connector, or a faulty control module. Proper electrical testing is required first.
Can I replace just the cylinder 8 glow plug?
While you can replace a single confirmed failed glow plug, many manufacturers recommend replacing all glow plugs as a set if they have high mileage, to prevent subsequent circuit codes from other cylinders.
Can I clear the P0678 code without fixing the problem?
Clearing the code only resets the warning light. If the underlying wiring fault, failed glow plug, or connector issue remains, the monitor will detect the electrical fault immediately and turn the light back on.
What Should You Do About the P0678 Code?
Confirm the exact factory definition and check for related glow plug codes for your specific vehicle application. Follow the applicable manufacturer diagnostic procedure to verify whether the fault lies in the cylinder 8 glow plug, the wiring harness, or the control module.
Once testing confirms the root cause, repair or replace the confirmed failed component in the relevant system and components, then retest the glow plug operation and clear the codes.
