P0678 DTC Glow Plug Circuit Cylinder 8
Content
P0678 DTC Glow Plug Circuit Cylinder 8
OBD-II DTC Datasheet
Glow plug chain of cylinder No. 8
What does this mean?
This Diagnostic Trouble Code (DTC) is a generic transmission code. It is considered universal as it applies to all makes and models of vehicles (1996 and newer), although the specific repair steps may differ slightly depending on the model.
This code refers to a device used by diesels to heat the cylinder head for a few seconds when trying to start a cold engine, called a glow plug. Diesel relies entirely on instant, high levels of compression heat to spontaneously ignite the fuel. The glow plug in cylinder # 8 is out of order.
When a diesel engine is cold, the extremely high air temperature caused by piston lift and air compression is quickly lost due to heat transfer to the cold cylinder head. The solution is a pencil-shaped heater known as a "glow plug".
The glow plug is installed in the cylinder head very close to the point that initiates combustion, or "hot spot". This can be the main chamber or the pre-chambers. When the ECM determines that the engine is cold using the oil and transmission sensors, it decides to assist the engine with starting with the glow plugs.
Typical Diesel Engine Glow Plug: 
It grounds the glow plug timer module, which in turn grounds the glow plug relay, which supplies power to the glow plugs. The module supplies power to the glow plugs. This module is usually built into the engine control computer, although it will be separate in cars.
Activating too long will cause the glow plugs to melt as they generate heat through high resistance and are red-hot when activated. This intense heat is quickly transferred to the cylinder head, allowing the combustion heat to retain its heat for the fraction of a second it takes to ignite the incoming fuel for start-up.
The P0678 code informs you that something in the glow plug circuit is malfunctioning causing the glow plug on cylinder # 8 to not heat up. To find a fault, you need to check the entire circuit.
Note: If DTC P0670 is present in conjunction with this DTC, run diagnostic P0670 before diagnosing this DTC.
symptoms
If only one glow plug fails, other than the check engine light coming on, symptoms will be minimal as the engine will usually start with one bad plug. In cold conditions, you are more likely to experience this. Code is the main way to identify such a problem.
- The engine control computer (PCM) will set code P0678.
- The engine will be difficult to start or may not start at all in cold weather or when it has been idle long enough to cool the unit.
- Lack of power until the engine has warmed up enough.
- Engine failure can occur due to lower-than-normal cylinder head temperatures.
- Motor may oscillate during acceleration
- There is no preheat period, or in other words, the preheat indicator does not go out.
Possible reasons
Reasons for this DTC may include:
- Defective cylinder # 8 glow plug.
- Open or short circuit in the glow plug circuit
- Damaged wiring connector
- Glow plug control module defective
Diagnostic steps and possible solutions
For a complete test, you will need a digital volt ohm meter (DVOM). Continue testing until the problem is confirmed. You will also need a basic OBD code scanner to restart your computer and erase the code.
Check the No. 8 cylinder glow plug by disconnecting the wire from the spark plug. Place the DVOM on ohm and place the red wire on the glow plug terminal and the black wire on a good ground. Range is 5 to 2.0 ohms (check measurement for your application referring to factory service manual). If it is out of range, replace the glow plug.
Check the resistance of the glow plug wire to the glow plug relay bus on the valve cover. Note that the relay (similar to the starter relay) has a large gauge wire leading to a bar to which all the glow plug wires are attached. Test the wire to the number one glow plug by placing the red wire on the number one bus wire and the black wire on the side of the glow plug. Again, 5 to 2.0 ohms, with a maximum resistance of 2 ohms. If it is higher, replace the wire to the glow plug from the tire. Also note that these pins from the busbar to the plugs are fusible links. Connect wires.
Check the same wires for looseness, cracks, or lack of insulation. Connect the code scanner to the OBD port under the dashboard and turn the key to the on position with the engine off. Clear codes.
Additional Resources P0678
We have found two helpful resources that can help you diagnose and troubleshoot DTCs. The first is a link to a great VW glow plug thread, the second is a video (we are not affiliated with any of the sources)
- Glow plugs 101 @ TDIClub.com
Related DTC discussions
- Chevy Duramax P2005 0678 Truck Won't StartRecently, water in the fuel caused the injector to knock and cylinder 3 misfires, the filter was changed, heat was added, and diesel was added. The truck sorted out and drove fine. Two days pass and the truck starts getting harder and harder to run the thrower code p0678. It's 80 degrees outside, and the truck wants to start. Looking at…
- 2008 Chevy Silverado 2500 code P0678Okay, I have a 2008 Chevy Silverado 2500. With the code P0678 I replaced the glow plugs 3 times (with the ones from oreilly) and replaced the glow plug control module (from the dealership) but the hazard code keeps returning. Any ideas? Thanks…
- P0678 Code on Chevy Silverado 3500 Duramax TruckI was told this code applies to cylinder number eight glow plug and / or chain. Can I drive the truck until a mechanic picks up the parts? Is there a chance that the truck will stop driving? ...
- 06 Silverado Diesel P0678Do I need to replace my glow plug or is there a way to check it? ...
Need more help with your p0678 code?
If you still need help with DTC P0678, post a question in the comments below this article.
NOTE. This information is provided for informational purposes only. It is not intended to be used as a repair recommendation and we are not responsible for any action you take on any vehicle. All information on this site is protected by copyright.
