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P0678 OBD-II Trouble Code: Cylinder 8 Glow Plug Circuit

Check Engine Light

P0678 code definition

The P0678 code detects an error with the glow plug circuit in cylinder 8

What the P0678 code means

The P0678 code is a generic OBD-II trouble code indicating a problem with the glow plug circuit in cylinder number 8. Glow plugs are used in diesel engines to help warm the engine to aid ignition. The powertrain control module (PCM) helps control this heat. When the voltage from the glow plug circuit in cylinder 8 is more than 10 percent off of the manufacturer’s suggested settings, then the PCM will be alerted, and the P0678 code will be detected.

What causes the P0678 code?

Many things can cause the P0678 trouble code to be stored, including:

  • Defective glow plugs
  • Defective glow plug relays
  • Malfunctioning glow plug timer or glow plug module
  • Blown fuses in the glow plug circuit
  • Issues with the electrical components in the glow plug circuit, such as open or short wires, or corroded, loose, or damaged connectors

What are the symptoms of the P0678 code?

The P0678 trouble code will usually be accompanied by the Check Engine Light, as well as the glow plug indicator warning light. The vehicle may take a while to start, especially in colder conditions. The vehicle may also experience white smoke emitting from the exhaust system during start-up.

How does a mechanic diagnose the P0678 code?

The P0678 code will be diagnosed using a standard OBD-II trouble code scanner. A reputable mechanic will use the scanner to assess the freeze frame data of the P0678 code, and look for any additional trouble codes that may be present. The mechanic will then reset the codes and restart the vehicle, to see if the code returns. If the code does not return, it is most likely an intermittent issue, or an incorrectly stored code.

If the P0678 code returns, then the technician will perform a visual inspection on the electrical components of the glow plug circuit. All wires, connectors, and fuses should be examined, and any that are damaged should be replaced.

Next, the mechanic can check to see if the glow plug is defective. This can be done using a digital volt/ohmmeter, or by hooking up a fully charged battery voltage to the glow plug on one end, and to the ground on the other end. If the glow plug is unable to heat up, or heats up to a bright red, then it is faulty and needs to be replaced.

If the issue is still unresolved, the relays, timers, and modules should be given thorough inspection.

Whenever a component is replaced, the trouble codes should be reset and the vehicle restarted. This way, the mechanic will be able to tell as soon as the issue has been fixed.

Common mistakes when diagnosing the P0678 code

The most common mistake made when diagnosing the P0678 codes comes from a failure to properly follow the OBD-II trouble code diagnosis protocol. The protocol should be followed exactly, to ensure that inspection and repairs are efficient.

It is common for timers and relays to be erroneously replaced, when the issue was the glow plug itself, or the electrical components.

How serious is the P0678 code?

If the P0678 code has been detected in the vehicle, it will still be drivable. However, there will be some issues, such as a prolonged start time, and issue should not be ignored.

What repairs can fix the P0678 code?

Repairs for the P0678 trouble code include:

Additional comments for consideration regarding the P0678 code

It is common for the P0678 code to be accompanied by other glow plug circuit trouble codes. When this is the case, codes should be inspected and repaired in the order that they appear in.

Since glow plugs are only used in diesel engines, the P0678 code will only apply to diesel vehicles.

Need help with a P0678 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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