How to Fix the P0678 DTC Code on Duramax (Expert Insights)

The Diagnosis Trouble Code or DTC is made to narrow down the exact issue caused by a vehicle from the abundance of other causes. If you have a Duramax truck, at some point, you need to decode P0678 to track down the issue and fix it conveniently.

DTC P0678 means the Glow Plug circuit in the Cylinder 8 of your Duramax is malfunctioning or has been damaged. A defect in the glow plug or the relay connecting the glow plug can cause the P0678 trouble code.

It is one of the less expensive issues that may happen in the engine of your Duramax. To learn more about the symptoms, causes, and fixes of P0678, you should go through this article entirely.

How to Fix the P0678 DTC Code on Duramax

What Does P0678 Code Mean On Duramax?

The DTC P0678 code means the Glow Plug located in the Cylinder 8 of the engine is either damaged or malfunctioning. When this Glow plug does not work properly, the engine fails to ignite due to a lack of initial heating.

Breaking down the DTC P0678 gives the elongated version of the issue address.

  • P = Powertrain ( compilation of the machines required for propelling the vehicle)
  • 0 = Generic (SAE) Standard Number
  • 6 = Computer Auxiliary Outputs
  • 78 = Specific Fault Index (In this case, Glow Plug of Cylinder 8)

There are many cylinders in the engine of the Duramax. Among them, the glow plug of cylinder 8 fails to work properly. Thus, it cannot warm up the engine to initiate the ignition sequence.

How Serious Is P0678 Code on Duramax?

The diagnostic trouble code P0678 is a serious issue for heavy-duty trucks. Especially if you reside in a very cold region, it can cause your engine not to start at all.

A damaged glow plug or the glow module causes the code P0678 in a Duramax truck. Thus you may see the white dense exhaust smoke driving with this issue. Moreover, bad glow plugs cause rapid misfires while accelerating.

Engine cylinders cannot get enough heat to cause combustion due to the faulty glow plugs. For this, you experience a downgrade in the vehicle’s overall driving performance.

This may cause the truck to be stalled on the road suddenly. Therefore, having a “Check Engine Light” turned On with P0678 can be a serious issue for your Duramax while going on a long journey.

What Causes P0678 Code On Duramax?

The primary reason for the P0678 trouble code is the issue with the glow plug cylinder 8, which is located on the Driver’s side. But along with that, there can be many other major issues incorporated, which results in the P0678.

  • Damaged or burned out Cylinder 8 Glow Plug
  • Shorted or burned out Glow Plug Circuit
  • Damage in the Glow Plug Control Module
  • Faulty or Broken Wiring
  • Defect in the Glow Plug Relay
  • Blown fuses of the glow plug circuit

How To Fix P0678 Code On Duramax?

Just like other DTC, you will need a portable OBB-II reader to initiate the fixing of the problem. Use this guide step by step to get rid of the Check Engine Light for P0678.

Confirming The Issue Type

At first, connect your OBD-Ii of any model to your truck’s scanner port located below the dashboard. Turn on the key but do not ignite the engine. In the engine standby condition, run the scanning in your OBD-II and wait for the result.

If the Glow plug of cylinder no 8 is faulty or part of the main issue, you will see the code P0678. Now there are two major solutions to this issue.

Solution 1: Replacing The Glow Plug

This solution involves the inspection and replacing of the entire glow plug. To do this, you require some disassembling, and hence some special tools may be required if you want to do it yourself.

  • Remove The Glow Plug

The glow plug in the Duramax is located on top of cylinder number 8 of the engine on the left driver’s side. Remove the fenders to get a good look at them. Use the specific socket to remove the Glow Plug.

  • Testing the Glow Plug

You will need a multimeter for this action. Connect the terminals of the multimer to check the resistance of the Glow plugs. The resistance on the glow plug should be less than 6 ohms.

If you find any deviation in the readings, the glow plug should be replaced. Besides, you can also see burned-out physical damages in the glow plug.

  • Replacing the Glow plug

Purchase the glow plug specific for this issue and test it before installing. Insert the glow plug in the port and lock it in place using the socket.

Solution 2: Replacing The Glow Plug Control Module

A Glow Plug Module makes a connection with the main engine computer system. It receives the reading from the PCM and transfers the command to the glow plugs to preheat the combustion chamber.

  • Remove and Inspect The Glow Plug Module

Firstly, remove the Glow Plug module from the inside hood of the Duramax. Perform a visual inspection of the glow plug module and check if other engine components cause damage or leaks.

You can also check the electrical connections for tearing or corrosion. If no damage is found, you need to shift to a different solution. But if you find any damage, it is recommended to replace the glow plug module.

  • Replace The Glow Plug Module

You need to purchase the specific model of the module for the Duramax Truck. The replacement process is very straightforward. All you need is to insert the Glow plug module in the specific port.

Other Symptoms Of Getting P0678 Code On Duramax

By knowing about the symptoms, you can easily confirm what is causing your truck to show P0678 DTC.

  • The Check Engine Light will be seen on the instrumental panel
  • Takes a lot of time for the engine to ignite in hot weather
  • The engine does not start in cold weather
  • Performance downgrading in acceleration and top speed
  • The leaking of diesel causes white smoke in the exhaust
  • Misfires in the combustion chamber of the engine

How Much Does It Cost to Fix Duramax P0678 Code?

There are two types of costs for troubleshooting the P0678 code. These are the product cost and the service cost. You can eliminate the service cost if you can manage the replacement of the products by yourself.

A glow plug module may cost you $375, and the glow plugs are around $50 each. Deepening upon the labor, you need to pay $90 to $200 for the replacement purpose.

Can You Drive Your Duramax With code P0678?

You can drive your truck with a broken glow plug on one cylinder. But it is not advisable because the engine can suddenly stall on the road. Besides, the engine may not even get started in cold regions, so you cannot drive the car.


It is important to know the health of your Duramax for a smooth and uninterrupted ride. We hope this article has enlightened you all about the causes, symptoms, and fixes of the DTC P0678.

This simple issue can be easily overcome by changing the glow plug or glow plug module. Thus, you need not pay the labor charge f you can manage the replacement yourself using some basic tools.

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