OBD-II code P0678 indicates an open circuit in the glow plug control or heating circuit for cylinder 8. This means the engine control module (ECM) has detected a fault preventing the glow plug from operating properly, which can affect cold-start performance and emissions.
The glow plug itself may have failed internally or developed an open circuit due to age, corrosion, or excessive heat cycles.
Wiring harness or connector issues (e.g., broken wires, corrosion, or loose connections) between the ECM and glow plug can cause an open circuit.
A blown fuse or faulty relay in the glow plug circuit can interrupt power delivery, triggering the code.
In vehicles with a dedicated glow plug control module, a failure here can prevent proper operation of the glow plug circuit.
A weak or corroded ground connection for the glow plug circuit can mimic an open circuit condition.
While uncommon, an internal ECM fault could theoretically cause a false open-circuit detection, but this is a last-resort diagnosis.
No. The ECM is rarely the cause of P0678. The issue is almost always a failed glow plug, wiring problem, or relay/fuse. Replace the ECM only after exhausting all other possibilities.
Driving is possible, but cold starts may be difficult, and prolonged operation with a faulty glow plug can lead to incomplete combustion, increased emissions, or potential damage to the catalytic converter.
Not necessarily. Glow plugs can be replaced individually, but if the vehicle has high mileage or other plugs show signs of wear, replacing them as a set may prevent future issues.
Glow plugs are primarily used during cold starts. The ECM may only detect the open circuit when the glow plug system is actively commanded on, which is more frequent in cold conditions.