Technical Description of OBD-II Trouble Code
Glow Plug Circuit for Cylinder #8
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may be slightly different depending on the model.
This code pertains to the component used in diesel engines to heat the cylinder head for a few seconds when starting a cold engine, known as a glow plug. In a diesel engine, the fuel is ignited by the momentary high level of heat generated through compression. However, the glow plug in the #8 cylinder has malfunctioned.
When a diesel engine is cold, the extremely high air temperature produced as the piston rises and compresses the air is quickly lost through heat transfer to the cold cylinder head. To address this issue, a pencil-shaped preheater called a “glow plug” is installed in the cylinder head near the combustion initiation point, also known as the “hot spot.” This can be in the main chamber or pre-chamber. When the engine management computer detects that the engine is cold based on oil and transmission sensors, it activates the glow plugs to assist with starting.
A typical diesel engine glow plug:
The glow plug timer module, which is grounded by the glow plug, activates the glow plug relay that supplies power to the glow plugs. The module is usually integrated into the engine management computer, although some vehicles may have it separate.
Activating the glow plugs for too long can cause them to melt due to the heat generated by heavy resistance, causing them to glow red hot. This intense heat is quickly transferred to the cylinder head, allowing the combustion heat to be maintained long enough to ignite the incoming fuel during starting.
The code P0678 indicates a fault in the glow plug circuit, resulting in the failure of the glow plug in cylinder #8 to heat up. The entire circuit must be inspected to locate the fault.
Note: If a P0670 trouble code is present along with this DTC, diagnose the P0670 code first before addressing this code.
Symptoms
With only one glow plug failure, aside from the check engine light illuminating, the symptoms will be minimal as the engine can still start with one faulty plug. However, these symptoms are more likely to occur in freezing conditions. The code is the primary means of identifying such a problem.
Potential Causes
Possible causes for this DTC include:
Diagnostic Steps & Possible Solutions
A digital volt ohm meter (DVOM) is required for the complete test. Follow the tests until the problem is confirmed. Additionally, a basic OBD code scanner is needed to reset the computer and clear the code.
To check the glow plug in cylinder #8, disconnect the wire connector on the plug. Set the DVOM to ohms and place the red lead on the glow plug terminal and the black lead on a good ground. The range should be between .5 to 2.0 ohms (refer to a factory service manual for the specific measurement for your application). If it is outside of this range, replace the glow plug.
Check the resistance in the glow plug wire to the glow plug relay buss bar on the valve cover. Note that the relay, which resembles a starter relay, has a large gauge wire leading to a buss bar where all the glow plug wires are connected. Test the wire connected to the number one glow plug by placing the red lead on the buss bar’s number one wire and the black lead on the glow plug side. Again, the resistance should be between .5 to 2.0 ohms, with 2 ohms being the highest resistance. If it is higher, replace the wire lead from the buss bar to the glow plug. Also, take note that these leads from the buss bar to the plugs are fusible links. Reconnect the wires.
Inspect the same wires for looseness, cracks, or missing insulation. Connect the code scanner to the OBD port under the dash and turn the key to the “on” position without starting the engine. Clear the codes.
Additional P0678 Resources
We found two helpful resources that may assist you in diagnosing and repairing your DTC. The first is a link to a helpful VW glow plug thread, and the second is a video (we are not affiliated with either source)
