How to Fix the P0677 Code On Duramax (Step-by-Step Guide)

If you have a Duramax engine, diagnostic trouble codes (DTC) can be a headache. However, DTC assists in analyzing the issue in the Duramax as early as possible to avoid any further damage.

The P0677 is defined as a malfunction in the glow plug circuit in cylinder Number 7 of a diesel engine like Duramax. When the Powertrain Control Module finds that the voltage reading from the glow plug in cylinder seven deviates from the manufacturer specification, it sets the P0677 code.

No worries, the P0677 error code is a relatively inexpensive one to fix.  This article will walk you through the symptoms, causes, and potential fixes of the codeP0677.

P0677 Code On Duramax

What Does P0677 Code Mean On Duramax?

The DTC P0677 means the Glow Plug in cylinder 7 of the Duramax diesel engine fails to preheat. The PCM detects the voltage reading of the plug, calculates the deviation, and sets the code.

You can see the breakdown formation of the P0677 below:

  • P = Powertrain, the system of components that propels your car forward.
  • 0 = a generic number derived from the SAE standard
  • 6 = Glow Plug
  • 77 = Specific fault index (it means a malfunction in the Glow Plug circuit in Cylinder Seven of the engine).

Glow plugs are installed in each cylinder to increase the temperature beforehand. When a Glow Plug is damaged, engines won’t ignite, especially in cold regions.

All in all, if the Glow Plug Control module malfunctions, the voltage reading sent to the integrated PCM deviates from the manufacturer’s specification.

How Serious Is P0677 Code On Duramax?

A Duramax engine shows no major driveability issues with the P0677 trouble code. However, if you live in an area where the weather is extremely cold, this can be a problem.

In such an environment, the engine fails to get started because cold accumulates in the head of the cylinder. It resists the necessary heat to be produced in the combustion chamber.

These can become a serious issue for the truck’s driveability and cause also cause damage.

What Causes P0677 Code On Duramax?

The primary cause for P0677 is the natural breakdown of the Glow Plug after a specific time of usage (60000 miles).

Apart from that, there are some other causes for the ECM of Duramax to show a P0677 trouble code, which are:

  • Glow Plug In-Cylinder Number Seven Is shorted or Damaged
  • High resistance in the Glow Plug than the normal range
  • Damaged Glow Plug Control Module
  • Damage in the metallic connector pins of the Glow Plug Control Module with the engine.
  • Broken wires, fuses of the PCM, and the Glow plug Control Module
  • Defect in the Glow Plug relays

How To Fix P0677 Code On Duramax?

To clear the code, the mechanic must first diagnose the code using a scanner and check the resistance using an ohm meter. Here’s a step-by-step guide to help you get through it quickly so you can get back to driving with your Duramax engine.

Step 1: Retrieving Freeze Frame Data

You need to possess a compatible OBD-II standard Scanner for this purpose. All Duramax engines produced after 2000 are compatible with the OBD coding for the diagnostic.

Connect your scanner to the diagnostic port located under the dashboard. The connecting port is located beside the gas/brake pedals in some trucks. Run a live scanning turning over the truck.

You can either write down the freeze frame data or save it in the scanner for future reference. Check if the other similar codes appear in the scanner related to glow plugs.

Step 2: Inspecting And Testing Glow Plug

For this, you would require some basic tools such as clamps and sockets to pull out the fenders under the engine cylinder. Cylinder number seven is located below the passenger side.

Remove the bolts and pull on the wheen fender to get a clear view of the Glow Plug. Take a multimeter and switch it to resistance testing mode.

Check the resistance of the Glow plug and note that the reading should be in the range between 0.5 to 2 ohms. If you find the result other than that, you need to replace the Glow plug.

Step 3: Inspection Of Other Electrical Connections

Use a multimeter to check the Glow Plug wire resistance to the glow plug relay bus bar. The optimum range for the resistance is also 0.5 to 2 ohm for the wires.

Use the Duramax Factory Service Manual for reference and take necessary actions to bypass the issue. Sometimes a loose connection can also set P0677; hence check all the wire connections are strong and tucked in.

For confirmation, reset the OBD-II tool while keeping the engine turned on and scan again after the reboot.

Step 4: Replacing Glow plug Module

The Glow Module is located on the engine and directly connected to the Engine Control Module (ECM).

Before approaching the replacement, it is imperative you should check the wires and electronic connectors for damage and contamination. If everything goes fine, replace the existing Glow Plug Module with a  new one compatible with the Duramax engine model.

Finally, reset the DTC codes in the scanner and turn over the engine to ensure the problem is fixed.

What Other Symptoms Of Getting P0677 Code On Duramax?

Having known about the symptoms of P0677 can save you some money from the initial diagnostic in the workshops. Some common symptoms of DTC P0677 are:

  • Check Engine Light will blink on the instrumental panel
  • The engine is not getting started
  • The engine will take a long time than usual to get started
  • Performance degradation of the engine, especially in acceleration and maintaining high speed.
  • Emission of white smoke from the exhaust

How Much Does It Cost To Fix Duramax P0677 Code?

For the initial diagnostic of the Duramax engine, you need to pay $75-$150 to the mechanic on an hourly basis. To replace the Glow Plugs, $10-$15 will cost each.

The entire fixation of the P0677 will cost you around $100-$200, including the placement charges.

Can You Drive Your Duramax With Code P0677?

You can drive the vehicle as long as one glow plug is defective. If more than one plug is faulty, the engine may be difficult to start.

Driving won’t be a major problem. However, if you let the issue go untreated for a long time, it may cause internal engine damage. That is why it is not recommended to drive a truck with P0677 unless you want to take it to a workshop to get it fixed.


Learning in detail about a DTC helps you avoid confusion and nervousness about your Duramax engine.

We hope this article enlightens you about the causes, symptoms, and finally, the probable fixes of DTC P0677. Now that you know all about P0677, it becomes easier to decide if you want to fix it in-home or workshop. Since it is a common issue with a simple fixing procedure, you can try to fix P0677 at home and save a lot of money.

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