The code P0678: Cylinder 8 Glow Plug Circuit indicates a fault in the glow plug circuit for cylinder 8 of the engine. This typically means that there’s an issue with the glow plug itself, the wiring to the plug, or the components controlling the plug, such as the relay or PCM. This fault can lead to difficulties in starting the engine, especially in colder temperatures when glow plugs are essential for diesel engine operation.
P0678 quick overview
| Meaning | P0678: Cylinder 8 Glow Plug Circuit |
| Is it serious? | This code indicates a specific issue with the cylinder 8 glow plug circuit and should be addressed promptly, especially in cold weather conditions, as it can affect engine starting and performance. |
| Possible causes | – Faulty or failed glow plug in cylinder 8 – Wiring issues or disconnections in the glow plug circuit – Problems with the glow plug relay or controller – Potential PCM (Powertrain Control Module) faults affecting the circuit |
| How to diagnose? | Inspect Glow Plug: Check the condition and functionality of the glow plug in cylinder 8. Examine Wiring and Connections: Look for any damaged or disconnected wiring in the glow plug circuit. Test Glow Plug Relay/Controller: Verify the operation of the glow plug relay or controller for proper function. Check PCM Operation: Consider potential issues with the PCM that might affect the glow plug circuit. |
P0678 Meaning
In simpler terms, the P0678 code means there’s a problem with the heating element (glow plug) in the eighth cylinder of your engine. This is like having a bad spark plug in a gasoline engine. The glow plug helps start your diesel engine, especially when it’s cold. If it’s not working right, you might find your vehicle hard to start on chilly mornings. The fix could be as simple as replacing the glow plug or fixing its wiring, or it might involve checking the parts that control the plug.
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How to fix P0678 – learn from mechanics
I looked into P0678 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.
ase #1: GMC
Vehicle Information: 2007 GMC Sierra 3500 HD WT 6.6L
Issue + Repair History: Initially showed codes P0678 and P0401. Fixed by cleaning EGR ports and changing glow plug #8. Then, code P0208 appeared.
Other OBD2 Codes + Test Results: P0208 – Short in injector
Suggestions to Fix Issue:
- Check the pins and harness for the injector, especially for wear or damage.
- Focus on areas disturbed during recent repairs, particularly around injector #8.
Final Fix: Resolved by reseating the plug that was not properly connected during the initial repair.
Case #2: Ford
Vehicle Information: 2004 Ford F-250 Super Duty King Ranch 6.0L
Issue + Repair History: Had codes P0672 and P0678 for glow plug issues. The truck had recent glow plug replacements and injector work done.
Other OBD2 Codes + Test Results:
- P0672: #2 glow plug issue
- P0678: #8 glow plug issue
- Tests showed good harness and ground connections
Suggestions to Fix Issue:
- Checking ohm values of individual glow plugs
- Swapping glow plugs to different cylinders and retesting
- Considering replacement of the glow plug control module (GPCM)
Final Fix: Replacing the GPCM resolved the fault codes and passed all tests.
Case #3: Ford
Vehicle Information: 2000 Ford Excursion Limited 7.3L
Issue + Repair History: Originally brought in for smog certification; failed due to incomplete monitors. Had multiple glow plug codes (P0603, P0672, P0676, P0678, P0683) after battery replacement. All glow plugs were replaced, and a Dorman 615-201 kit was used to address damaged wires at the valve cover. Codes P0676, P0678, and P0683 persisted.
Suggestions to Fix Issue:
- Checking resistance of pink/orange wire from the green connector at GPCM to PCM.
- Inspecting for burnt pins and corrosion in connectors.
- Considering replacement of the glow plug control module (GPCM).
- Checking for loose connections at the green connector.
Final Fix: After consulting with Identifix Hotline, a remanufactured Cardone module was identified as faulty. Replacing the module with a new one from WorldPac resolved the issue, along with addressing a broken locking tab on the green connector.
Case #4: Ram
Vehicle Information: 2017 Ram 1500 Big Horn 5.7L
Issue + Repair History: Initially presented with SES light on and fault codes P0678, P1206, and P025A. After replacing the fuel pump assembly and the fuel pump control module, the same codes returned. The fuel pressure varied from 38psi to 75psi KOER before settling between 60-70psi.
Suggestions to Fix Issue:
- Inspect the fuel pump control module connectors for corrosion or loose terminals.
- Check wiring harness for possible chafing, especially near the frame rail under the driver’s front inner fender liner.
- Load test circuits and verify proper power and ground supply to the fuel pump control module.
- Consider potential TIPM issues or PCM reflash/update if applicable.
Final Fix: Identified a wiring harness issue where the driver’s front inner fender liner rubbed through the wire harness on the frame rail. Repairing the wiring harness resolved the fault codes and corrected the issue.
Case #5: Chevrolet
Vehicle Information: 2008 Chevrolet Silverado 3500 HD LT 6.6L
Issue + Repair History: The truck intermittently lost throttle response with DTC P0652 set. The 5V reference voltage dropped to 0V during these episodes. The issue was traced to a short in the 5V reference “2” circuit, affecting multiple sensors. The truck had undergone modifications including DPF removal.
Other OBD2 Codes + Test Results:
- P0652: 5 Volt reference 2 Circuit Low Voltage
- P0193: Fuel Rail Pressure Sensor
- P0678: Cylinder #8 Glow Plug Circuit/Open
Suggestions to Fix Issue:
- Place inline fuses on each 5V ref “2” circuit and monitor which fuse pops.
- Inspect the FRP sensor and wiring due to the P0193 code.
- Check for corroded terminals or disconnections, especially if the truck has modifications like DPF removal.
- Look for wiring chafes near the oil pressure sensor or along the transmission case.
Final Fix: The issue was resolved by repairing a short in the 5V wire going to the EOP sensor. The EOP sensor was disconnected, leading to the stabilization of the 5V reference. Replacing the EOP sensor ultimately fixed the vehicle.
How to diagnose P0678
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.
| Advice | Description |
|---|---|
| Inspect Glow Plug and Wiring | Check the glow plug for proper operation and inspect its wiring for any damage or disconnections. |
| Test Glow Plug Resistance | Measure the resistance of the glow plug to ensure it falls within the specified range. |
| Examine Connector and Harness | Look for any corrosion, loose connections, or damage in the glow plug connector and wiring harness. |
| Check for ECM/PCM Issues | Consider the possibility of a malfunction in the Engine Control Module or Powertrain Control Module affecting glow plug operation. |
| Investigate Power and Ground Circuits | Ensure the glow plug is receiving proper power and has a good ground connection. |
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