Error Code P0670 is defined as Glow Plug Module Control Circuit. This code means there’s a problem with the Glow Plug Control Module, which often results from poor electrical connections or defective components.

This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, especially vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repairs vary from one vehicle make and/or model to another. It encompasses all brands of late model diesel engines, including diesel vehicles from Chevrolet, Dodge, Ford, GMC and VW vehicles.


Unlike a gas engine, a diesel engine doesn’t depend on compressed fuel mixture and electrical source of ignition to work. This is because diesel has higher compression than gas engines.

High compression causes the air inside the cylinder to reach more than 600˚ C, enough to ignite diesel fuel. When the piston reaches top dead center of the cylinder, high-pressure fuel sprays into the cylinder. It quickly ignites when it encounters super-heated air to ignite the fuel, when the engine is cold a problem will rise. A cold start makes it difficult for the engine to super-heat the air, as heat is quickly lost due to the cold cylinder head.

This is where the job of the glow plug gets into the picture; installed in the cylinder, this pencil-shaped component heats up and glows up for about 10 seconds. This results in an increase in the temperature of the surrounding wall, allowing heat compression to rise for the ignition.

If the glow plug is not working properly, the engine may not start. And this is what the Error Code P0670 is about.

Common Symptoms

Since this error code refers to the glow plug, it will cause the glow light to illuminate, causing the engine to start slowly in warm weather, or fail to start in cold weather. Or at the very least, the engine hesitates before starting.

If the engine is able to start, pronounced knocking will be evident until the engine is able to warm up to the right operating temperature. White smoke will also come out of the exhaust, which is a result of excess fuel from hard start burn offs.

The engine will also have noticeable miss until the temperature of the cylinder head rises to the a sufficient level to support complete combustion.

Possible Causes

Error Code P0670 is caused by multiple of factors, but the most common is in the Glow Plug Control Module (GPCM) itself, which sets off the code. Other common causes include:

  • Shorted or open GPCM harness
  • Poor electrical connections in the GPCM harness
  • Malfunctioning PCM
  • Poor injection timing in glow plug caused by wear and tear

How to Check

To diagnose this code, technicians start with using an OBD-II scanner to read the codes that have been stored.

Next, they proceed to check the battery if it’s working.

Then, they’ll inspect the wiring and look for any signs of defects that may lead to GPCM not getting the right level of power it needs to function properly. This is where they will also use a voltmeter.

Next, they will check if the ignition switch delivers power. They will conduct the same with the relay actuation by simply disconnecting the oil temperature sensor and then hitting the ignition.

How to Fix

Repairs for this error code are as easy as its diagnosis. Mechanics usually do the following:

  • Change the battery
  • Replace damaged connectors and wires
  • Replace the GPCM
  • Replace the glow plug relay
  • Replace the PCM (rare condition)

Error Code P0670 will neither cause safety concerns for the passengers of the vehicle, nor will cause serious damage to the vehicle. But until it’s fixed, you will experience a terrible time with your ignition with this error code. Thus, it’s important that it’s addressed ASAP.