Error Code P0673 is defined as Cylinder 3 Glow Plug Circuit. This means there’s an issue with a computer output circuit caused by a defective glow plug in cylinder 3, in a relay and a module or timer.

This error code is generic, meaning it applies to all vehicles equipped with OBD-II system. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another.


Unlike a gasoline engine, a diesel engine doesn’t rely on compressed fuel mixture and electrical source for the engine’s ignition to work. This is because the latter has higher compression. It’s what helps the air inside the cylinder reach more than 600˚ C, which is needed to ignite the diesel fuel. This high pressure fuel is then sprayed into the cylinder when the piston reaches top dead center in the cylinder. Then, it quickly ignites when it encounters super-heated air, igniting the fuel.

When the engine is cold a problem will rise. Because heat is quickly lost due to cold cylinder head (during cold temperature), diesel engines will have a difficult time to super-heat at cold temperature.

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 to an increase of temperature of the surrounding wall, allowing heat compression to rise for the ignition.

Error Code P0673 happens when there’s a problem in the glow plug circuit, causing cylinder #3 glow plug to fail from heating up. This will require a complete and thorough inspection to locate the fault.

Common Symptoms

Since this code means one bad plug, the engine can normally start with minimal symptoms. In this situation the most common symptom would be the illumination of the Check Engine light. For vehicles which are freezing, there could be more symptoms, such as:

  • Hard starting or no start condition when sitting long enough
  • Engine lacks power until it heats up completely
  • Engine hesitates when accelerating
  • Engine miss due to cylinder head temperature being colder than normal
  • No pre-heat period, Pre-heat light doesn’t go off

Possible Causes

There can be multiple of factors to contribute this error code, such as:

  • Faulty cylinder #3 glow plug
  • Damaged wiring connector
  • Open or shorted glow plug circuit
  • Faulty glow plug control module

How to Check

To diagnose this code, technicians start by using a DVOM (digital volt-ohmmeter). They disconnect the glow plug’s connector, set the DVOM on ohms and place the red lead on the terminal of the glow plug and at the back end of a good ground. The range should be between 0.5 to 2.0 ohms. They verify the measurements from the factory service manual. If the range is outside the specifications, then the glow plug must be replaced.

Next, they check the resistance in glow plug wire to the glow plug relay buss bar on the valve cover. Notice how the relay (which looks like a starter relay) has large gauge wire that leads to a buss bar where all the wires of the glow plugs are attached.

Next, they test the wire to the #3 glow plug by replacing the red lead on the buss bar # wire and the black lead to the glow plug inside. Once again, the range must be 0.5 to 2.0 ohms (with 2ohms being the highest resistance). If the resistance is higher, then the wires from the buss bar to the glow plug must be replaced. They make sure to reconnect wires, as this may cause the buss bar to splurge.

Then, technicians look for signs of cracks, looseness, or missing insulation. Lastly, they connect the code scanner to the OBD port on the vehicle (usually located under the dash), turn the key at “on” with the engine off. Then they clear the codes.

How to Fix

Common repairs for this error code are of course, derived from the diagnosis. Examples of the repairs include:

  • Repair or replace any damaged, corroded, or loose wiring, connectors, and harnesses
  • Replacing blown fuses and repair shorts
  • Repair or replace damaged or bent pins in electrical connectors
  • Replacing the cylinder #3 glow plug
  • Replacing the glow plug relay

Note that, if P0670 trouble code is present along with this DTC, it is important to diagnose P0670 before diagnosing this code.