P0678 – Glow plug, cylinder 8 -circuit malfunction

Tapley Mitchell

By Tapley Mitchell (Contact Me)
Last Updated 2019-09-25
ASE Mechanic

CodeFault LocationProbable Cause
P0678 Glow plug, cylinder 8 -circuit malfunction
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Wiring, poor connection, relay, glow plug control module, glow plug, ECM

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What Does Code P0678 Mean?

This code indicates that there is a failure in the glow plug circuit preventing the glow plug form operating normally. In a Diesel engine, there is no spark to ignite the air/fuel mixture. Instead the heat from compressing the mixture rapidly is enough to ignite the air/fuel mixture. Diesel engines need to first reach a good operating temperature before this is possible. Before the engine is warm it uses a glow plug to heat up the cylinder to enable the combustion of the air/fuel mixture. Without a glow plug combustion may not be possible in cooler temperatures.

What are the symptoms of code P0678 ?

  • Long crank times
  • Lower power during engine warmup
  • Difficulty starting when cold
  • Check engine light – on
  • Longer than normal glow plug times

How do you troubleshoot code P0678 ?

Before you make the most common mistake of purchasing a new glow plug without verifying the problem, let’s take a look at the entire system. Since this code is specific to cylinder 8 you will need to determine which cylinder that is. (this may be different depending on the manufacturer of the vehicle).

Once you have located which cylinder is causing the issue, find a wiring diagram for your vehicle that includes the circuit for the glow plugs. Check to see if there are relays and timers for each individual cylinder. Some engines have one timer and relay for each cylinder, while the other will have one timer per bank and a relay for each cylinder, and still some that have only one timer and relay per bank. If you have a timer and relay for each cylinder they can be a part of the cause.

The fastest way to determine if the issue is the glow plug itself, if there are no obvious wiring damages, is to swap the glow plug in question for another cylinder glow plug. Clear the codes in the ECU and see how the problem returns. If the code changes to the cylinder you have moved the plug to, then it’s most likely that the plug itself needs replaced.

If you switched the plug to another cylinder and still get the same code, then the plug is likely not the problem. The problem in this case will be in the wiring, or control components. Start by looking at the wiring very carefully and repairing any damage such as corrosion, broken wires, damaged insulation, or any other flaw you may find with the wiring. Next check the relay to make sure that it is operating correctly and has a good connection. Follow this step by verifying that there is proper voltage to the relay. You should see constant battery voltage on the load side, while the control side should be present when the glow plug light on the dash is on. If you have low, or no voltage here it could be the controller or the timer.

If the timer is cylinder specific, you may be able to swap it with another cylinder timer and see if the code moves with it. If you get a different code that moved with the timer, then it is most likely the timer, if the code does not move then it will most likely be the controller that has had a fault. You can verify this by finding the pin that goes to cylinder 8 glow plug circuit and see if there is an output signal when the glow plug light is on in the dash. If the pin is dead while all other pins have voltage, then the controller is your problem.

Codes Related to P0678

If you have other codes that are related to this code it may be a good indicator of where to start looking for the problem. If you have code P0670, that code will need to be repaired before you continue onto repairing this code. If you have any of the codes P0671-P0678, the procedure to diagnose, and repair will be similar.