The P0674 code is a generic OBD-II diagnostic trouble code indicating “Cylinder 4 Glow Plug Circuit.” It means the powertrain control module detected an electrical fault in the glow plug circuit for the fourth cylinder.
This code does not automatically prove the glow plug has failed. Diagnosis requires testing the cylinder 4 glow plug, inspecting the wiring, and verifying the control module before replacing any components.
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Key Takeaways
- The P0674 code indicates the control module detected an electrical fault in the glow plug circuit for cylinder 4.
- Possible causes include a failed cylinder 4 glow plug, damaged wiring, corroded connectors, or a failed glow plug control module.
- Diagnosis involves swapping the glow plug to another cylinder, measuring resistance, and checking the circuit voltage.
- Repairs may involve replacing the cylinder 4 glow plug, repairing the wiring harness, or replacing the control module.
P0674 Code Description and Implications
The P0674 code indicates that the powertrain control module or glow plug control module detected an abnormal voltage or resistance in the cylinder 4 glow plug circuit. This generic error code applies to diesel engines.
Glow plugs heat the combustion chamber to assist with cold starting. The control module monitors each circuit to ensure the glow plugs are functioning correctly before and during engine cranking.
When the module detects an open circuit, short to ground, or out-of-range resistance specifically on cylinder 4, it stores this code and may illuminate the glow plug warning light.
Common Causes of the P0674 Code
A failed glow plug in cylinder 4 is a primary cause. The internal heating element can burn out or break, creating an open circuit condition.
Damaged wiring, chafed insulation, or corroded connectors in the harness between the control module and the cylinder 4 glow plug will interrupt the control signal.
An internally failed glow plug control module or powertrain control module driver can prevent voltage from reaching the cylinder 4 glow plug, triggering the circuit fault.
Symptoms Associated With P0674
An illuminated Check Engine Light or specific glow plug warning indicator is expected once the monitor detects the circuit fault. MIL behavior varies by application.
You may experience extended cranking times or hard starting, particularly in cold weather, because cylinder 4 lacks the necessary pre-heating for proper combustion.
The engine may idle roughly or produce white smoke from the exhaust during cold starts due to incomplete combustion in the fourth cylinder.
Diagnosing the P0674 Trouble Code
Diagnosis must begin by confirming the exact factory definition and checking for related glow plug or module codes. Record freeze-frame data to see the conditions when the code set.
A practical test involves swapping the cylinder 4 glow plug with one from another cylinder. Clear the codes and run the engine. If the code moves, the glow plug is faulty.
If the code remains P0674, use a multimeter to test the wiring harness for continuity, shorts to ground, and open circuits. Verify that the control module is supplying the correct voltage.
Repairing the P0674 Issue
Repairs must follow confirmed diagnosis. If the swap test or resistance measurement confirms the cylinder 4 glow plug is internally open or failed, replace it using a component meeting the exact vehicle specification.
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 glow plug and wiring are functional but no voltage is present, replace the glow plug control module or follow the manufacturer procedure for the powertrain control module.
Cost Considerations for P0674 Repairs
Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.
Replacing a single glow plug involves minimal parts cost, but labor can increase significantly if the plug is seized or requires cylinder head removal for extraction.
Replacing the glow plug control module involves higher parts costs and may require programming or adaptation with a scan tool.
Prevention Strategies for Future Occurrences
Replacing glow plugs as a complete set when one fails can prevent subsequent circuit codes and ensure balanced cold-start performance across all cylinders.
Using the correct diesel fuel and anti-gel additives in freezing temperatures helps prevent excessive cranking and strain on the glow plug system.
Some electrical failures occur due to normal component aging and cannot be reliably prevented through standard maintenance.
Troubleshooting Tips and Best Practices
Do not apply continuous 12-volt power directly to a modern glow plug for testing. Many modern diesel engines use regulated glow plugs that can overheat and fail instantly without the module’s pulse-width modulation.
Always verify the exact glow plug part number. Using an incorrect resistance or voltage rating can cause immediate module faults or premature plug failure.
Check for technical service bulletins for your exact vehicle application, as some manufacturers release updated glow plug designs or specialized extraction tools to address chronic breakage issues.
More OBD-II Codes
| P0651 | P0652 | P0654 | P0657 |
| P0660 | P0661 | P0675 | P0676 |
| P0677 | P0678 | P0683 | P0685 |
| P0671 | P0672 | P0673 | P0688 |
| P0690 | P0691 | P0700 | P0705 |
| P0715 | P0720 | P0730 | P0740 |
Frequently Asked Questions
Is the P0674 code generic or manufacturer-specific?
Yes, P0674 is a generic SAE OBD-II diagnostic trouble code. However, the exact glow plug testing procedures and control module designs vary by manufacturer and diesel engine application.
Will a P0674 code prevent my diesel engine from starting?
It can cause hard starting. The engine may still start, but it will likely crank longer and run rough until it warms up, because cylinder 4 is not being pre-heated for combustion.
Does a P0674 code mean I need to replace all glow plugs?
Not necessarily. The code specifically identifies a fault in the cylinder 4 circuit. However, if the plugs have high mileage, replacing them as a set is often recommended to prevent future failures.
Can I test a glow plug by jumping it directly to the battery?
No. Many modern diesel glow plugs are self-regulating or require pulse-width modulated voltage. Applying continuous 12 volts can instantly burn out the heating element. Use a multimeter to measure resistance instead.
Can I clear the P0674 code without fixing the problem?
Clearing the code only resets the warning light. If the underlying open circuit, failed glow plug, or damaged wiring remains, the monitor will detect the fault immediately and turn the light back on.
What Should You Do About the P0674 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 4 glow plug, wiring, or control module.
Once testing confirms the root cause, repair or replace the confirmed failed component in the relevant system, then retest the cold start operation to ensure the glow plug circuit functions correctly.
