The OBD2 code P0674 is specifically related to an issue with the glow plug circuit in cylinder #4, indicating a malfunction in the system that preheats the air in the cylinder of a diesel engine to facilitate starting in cold conditions. This code suggests that there might be an electrical fault, such as a poor connection, a faulty glow plug, or problems with the control circuit, affecting the engine’s ability to start or run smoothly in cooler temperatures.
P0676 quick overview
| Meaning | P0674: Glow Plug Circuit Fault in Cylinder #4 |
| Is it serious? | Yes, especially in cold weather, as it can impact engine start-up and performance. |
| Possible causes | – Faulty glow plug in cylinder #4 – Issues with the glow plug harness or connector – Malfunctioning glow plug relay or control module – Wiring problems affecting the glow plug circuit |
| How to diagnose? | Inspect the glow plug in cylinder #4 for damage or wear Check the glow plug harness for low clamping pressure or poor connections Measure current draw of the glow plug to ensure it’s within specifications Replace all glow plugs simultaneously if one is found to be faulty Examine the glow plug control unit and wiring for any signs of malfunction |
P0676 Meaning
For those unfamiliar with diesel engines, think of glow plugs as small heaters that warm up the engine’s cylinders to make starting easier, especially when it’s cold. The P0674 code indicates a problem with the heater in cylinder #4, which could mean the plug itself is bad, the wires connected to it are damaged, or the system controlling the plug isn’t working right.
It’s like having a faulty heating element in an electric kettle; without it functioning properly, getting the water (or in this case, the engine) to the right temperature can be difficult.
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How to fix P0676 – learn from mechanics
I looked into P0676 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.
Case #1: Volkswagen
Vehicle Information 2002 Volkswagen Jetta TDI 1.9L
Issue + Repair History
- Initially diagnosed with code P0674 for glow plug #4.
- Replaced glow plug #4, but problem persisted.
- Code P0674 (glow plug #4) reappeared after two weeks.
Other OBD2 Codes + Test Results
- P0674 (glow plug #4).
Mechanic Suggestions
- Check the glow plug harness for low clamping pressure causing poor connection to the glow plug. Replacement of the glow plug harness is recommended.
- Verify current draw at all glow plugs and ensure it does not exceed 2 amps after the reading settles.
- Consider replacing the glow plug control unit along with the glow plug harness.
- Replace all glow plugs simultaneously to prevent faults from recurring.
- Inspect for loose connections or corrosion inside the bridge connector; replacement of the glow plug bridge harness may be necessary.
- Replace the glow plug harness with a short pigtail connector to address potential wiring issues.
- Always replace all four glow plugs simultaneously and check the relay and wiring harness for potential issues.
- Look for updated TSBs related to the glow plug harness and ensure that all glow plugs have consistent resistance.
- Verify the integrity of the glow plug harness, especially the black connectors, and remember that glow plug #4 is actually in cylinder #1.
Final Fix
- Replaced glow plug bridge harness and all four glow plugs, cleared all codes, and verified the problem was resolved. No check engine light appeared after three days of driving.
Case #2: Volkswagen
Vehicle Information 2002 Volkswagen Jetta TDI 1.9L
Issue + Repair History
- Scan revealed fault code P0674.
- Alldata search yielded no results for this code.
- Code persists after clearing and returns after one test drive.
- No noticeable performance issues except for the illuminated MIL.
Other OBD2 Codes + Test Results
- P0674 (glow plug #4).
Mechanic Suggestions
- Verify the glow plug and its harness for any electrical malfunction.
- Replace the glow plug and harness if necessary.
- Check for consistent resistance among all glow plugs; replace the glow plug relay (controller) if needed.
- Utilize a current pickup clamp to measure amp readings of each glow plug.
- Consider updating the glow plug controller under the wiper cover on the driver’s side.
- Consult resources like Autotap.com for additional code information.
Final Fix
- The customer opted to repair the issue independently by replacing glow plug #4 and utilizing additional glow plugs available at home. The recommendation to use the iATN tool for code lookup proved beneficial in identifying the problem.
Case #3: Ford
Vehicle Information 2004 Ford F-350 Super Duty Harley-Davidson Edition 6.0L
Issue + Repair History
- Codes for all 8 glow plug circuits detected.
- Glow plugs and module confirmed as good; module is new.
- Initial current draw on each glow plug starts at 20-22 amps and drops thereafter.
- No wiring in the valve cover gaskets.
- 12V confirmed at each glow plug.
Other OBD2 Codes + Test Results
Mechanic Suggestions
- Check valve cover harness for shorts.
- Inspect injector harness (part of valve cover gasket) if glow plugs are under valve covers.
- Verify power supply to glow plug controller.
- Check amp draw of each glow plug.
- Consider TSB for updated glow plug module wiring harness connector.
- Examine wiring feeding B+ to plugs and control module for issues.
- Investigate ground problems, particularly on the ground side of the controller or PCM.
- Check harnesses at the controller for pushed out pins.
- Assess fuel injector control module (FICM) for codes/data and inquire about the use of starting aids.
Final Fix
- Corrosion was found in the main power wire connected to the right battery cable. After replacing the cable end and cleaning the battery terminals, the issue was resolved.
How to diagnose P0676
I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.
| Diagnostic Step | Explanation |
|---|---|
| Inspect glow plug in cylinder #4 | Check for physical damage or signs of wear. |
| Check the glow plug harness | Look for issues like low clamping pressure or damaged connectors. |
| Measure current draw of the glow plug | Ensure it does not exceed specified limits after settling. |
| Replace all glow plugs simultaneously | To prevent future faults and ensure uniform performance. |
| Examine the glow plug control unit and wiring | Check for any signs of malfunction or damage. |
Addressing a P0674 code involves careful inspection and testing of the glow plug system to pinpoint the exact cause of the fault. Given the importance of glow plugs in ensuring smooth engine starts in cold weather, resolving this issue promptly can help maintain optimal engine performance and reliability. Always consider professional assistance for accurate diagnosis and repair, especially in complex electrical systems.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko