P0678 Glow Plug Circuit DTC, Cylinder No. 8

P0678 – OBD-II Trouble Code Technical Description

Glow plug chain for cylinder No. 8

What does the fault code mean P0678?

DTC P0678 is a universal code that applies to all makes and models of vehicles from 1996 onwards. It is related to the operation of the glow plug in diesel engines. When a diesel engine is cold, the glow plug supplies additional heat to ensure starting. The glow plug located in cylinder #8 is not functioning properly.

The role of the glow plug is to provide sufficient heat to begin combustion of fuel in a cold engine. This occurs due to the strong resistance inside the candle, which creates heat. If the glow plug is not working, it can cause difficulty starting the engine, especially on cold days.

Code P0678 indicates a fault in the cylinder #8 glow plug circuit. To eliminate this malfunction, it is necessary to diagnose the entire circuit, including the wiring and glow plug. If the P0670 code is also present, it is recommended that you start by diagnosing it.

Possible reasons

Reasons for this DTC may include:

1. Defective cylinder # 8 glow plug.
2. Open or shorted glow plug circuit.
3. Damaged wiring connector.
4. The glow plug control module is faulty.
5. Insufficient power or grounding of the glow plug.

What are the symptoms of a fault code? P0678?

If just one glow plug fails, other than the check engine light coming on, symptoms will be minimal because the engine will usually start with one faulty plug. In frosty conditions you will be more likely to experience this. Code P0678 is the main way to identify such a problem, and includes the following symptoms:

1. The engine will be difficult to start or may not start at all in cold weather or after being parked for a long time when the unit has cooled.
2. Lack of power until the engine has warmed up enough.
3. Engine failure can occur due to lower-than-normal cylinder head temperatures.
4. The engine may hesitate when accelerating.
5. There is no preheat period, or in other words, the preheat indicator does not go off.

Code P0678 is important to diagnose and repair to ensure proper diesel engine operation, especially in cold conditions.

How to diagnose a fault code P0678?

To fully test and diagnose the glow plug and related components, you will need the following tools and steps:

Tools:

1. Digital Volt-Ohm Meter (DVOM).
2. Basic OBD code scanner.

Steps:

1. Disconnect the wire connector from the cylinder #8 glow plug.
2. Using a digital volt-ohm meter (DVOM), set it to resistance mode. Insert the red wire to the glow plug terminal and the black wire to a good ground.
3. Check the resistance of the glow plug. The resistance range should be between 0,5 and 2,0 ohms (check the measurement for your specific vehicle, per the factory service manual). If the measured resistance is outside this range, the cylinder #8 glow plug is faulty and requires replacement.
4. Check the resistance of the wire from the glow plug to the glow plug relay bus on the valve cover. Again, use the volt-ohmmeter and measure the resistance in this wire. It should also be in the range of 0,5 to 2,0 ohms.
5. Note that the glow plug relay looks like the starter relay and has a larger gauge wire leading to the bus bar that all the glow plug wires are connected to.
6. If the wire resistance is outside the specified range, replace the wire.
7. Check all wires for loose, cracked or missing insulation. Replace any damaged wires.
8. Reconnect all wires to the glow plugs and make sure the connections are secure.
9. Connect the code scanner to the OBD port under the dash and turn the key to the "on" position with the engine off.
10. Use the scanner to clear error codes (if they have been stored). This will clear the P0678 code and allow you to test with a clean slate.

These steps will help you diagnose and correct problems with the #8 cylinder glow plug and related components, ensuring proper diesel engine operation.

Diagnostic errors

Mechanical errors when diagnosing code P0678 (Cylinder No. 8 Glow Plug Malfunction) may include:

1. Not knowing how glow plugs work: A mechanic may not know how glow plugs work in diesel engines or how to test them. This can lead to undiagnosed or misdiagnosed problems.
2. Not using the right tool: Diagnosing glow plugs and related components requires a digital volt-ohm meter (DVOM) and sometimes an OBD code scanner. The absence of this tool can make correct diagnosis difficult.
3. Faulty Parts: A mechanic may skip diagnosing and replacing faulty glow plugs or wires, causing the problem to persist.
4. Faulty Glow Plug Relay: If a mechanic does not check the glow plug relay and replace it if necessary, this could also be a fault.
5. Incorrect glow plug life: Glow plugs have a limited life and require periodic replacement. If a mechanic does not take this factor into account, he may underestimate the cause of the problem.
6. Failure to Clear DTCs: If a mechanic does not clear DTC P0678 after repair work has been performed, the Check Engine Light will remain active, which can be confusing to the vehicle owner.
7. Insufficient Inspection of Related Components: In addition to the glow plugs, it is also important to inspect the wires, relays, and other components associated with this system. Unaccounted for problems with these parts may cause repeated failure.

To avoid these errors, mechanics should have a good understanding of the glow plug system, use the correct diagnostic tool, be diligent in inspecting and servicing related components, and properly clear error codes after performing repair work.

How serious is the fault code? P0678?

Trouble code P0678, which indicates a problem with the glow plugs of cylinder No. 8 in a diesel engine, can be considered serious. This code indicates a potential problem that may make the engine difficult to start and operate, especially in cold conditions.

Glow plugs in diesel engines play a key role in preheating the air in the cylinder before starting. If the #8 cylinder glow plug is not working properly, it can cause difficult starting, poor performance, poor fuel economy, and even long-term engine damage.

Therefore, if you have a P0678 code, it is recommended to have it diagnosed and repaired as soon as possible to avoid serious engine performance problems. This is especially important during cold weather, when a well-functioning glow plug system can be critical to the vehicle's successful starting.

What repair will help eliminate the code? P0678?

The following repairs will be required to resolve DTC P0678, which is a cylinder #8 glow plug problem in a diesel engine:

1. Cylinder #8 Glow Plug Replacement: The first step should be to replace the glow plug itself as it is the main cause of this problem. Make sure the spark plug you choose meets your vehicle's specifications.
2. Glow Plug Wire Inspection and Replacement: The wire connecting the cylinder #8 glow plug to the relay or glow plug control module must be checked for continuity. If damage is found, the wire should be replaced.
3. Replacing the Relay or Glow Plug Control Module: If the problem persists after replacing the plug and wire, you should check the relay or glow plug control module. If these components fail, they must be replaced.
4. Checking the bus and connections: It is also worth checking the condition of the bus to which the glow plugs are connected and all connections to ensure their integrity. Damaged connections should be replaced or repaired.
5. Re-Diagnose and Clear Code: After all necessary repairs have been made, the system should be re-diagnosed using a code scanner and, if necessary, clear the P0678 code.

Please note that in order to successfully repair and resolve the P0678 code, it is important to use quality and appropriate parts, as well as perform a system performance check after repair to ensure there are no problems.

What is P0678 Engine Code [Quick Guide]

P0678 – Brand-specific information

Information about the P0678 trouble code may vary depending on the specific vehicle brand. Below is a list of some car brands and their meanings for the P0678 code:

1. Ford: P0678 - Glow Plug Circuit, Cylinder 8 - Voltage Low.
2. Chevrolet: P0678 – Cylinder #8 Glow Plug – Voltage Low.
3. Dodge: P0678 - Glow Plug Monitor, Cylinder 8 - Low Voltage.
4. GMC: P0678 – Cylinder #8 Glow Plug – Voltage Low.
5. Ram: P0678 – Glow plug monitoring, cylinder 8 – low voltage.
6. Jeep: P0678 - Glow Plug Monitor, Cylinder 8 - Low Voltage.
7. Volkswagen: P0678 – Glow plug, cylinder 8 – low voltage.
8. Mercedes-Benz: P0678 – Glow plug control circuit, cylinder 8 – low voltage.

Please refer to the service and repair manual for your specific vehicle brand or your authorized brand representative for more detailed information and recommendations on how to resolve this problem.