Error Code P0679 is described as Cylinder 9 Glow Plug Circuit. This code means there’s a problem with the computer output circuit caused by a defective glow plug in cylinder number 9, 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 model to another.
Diesel engine, unlike gasoline, doesn’t need compressed fuel mixture and an electrical source for its ignition to work. This is because diesel engine has higher compression, which helps the air inside its cylinder reach 600˚ C, adequate to ignite 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.
In cold weather, the engine may have difficulty starting, as heat is quickly lost inside the cold cylinder head. With this, a diesel engine will have a difficult time to super-heat. 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 of temperature of the surrounding wall, allowing heat compression to rise for the ignition.
Error Code P0679 happens when there’s a problem in the glow plug circuit, causing the cylinder #9 glow plug to fail from heating up. This will require a complete and thorough inspection to locate the fault.
Since this code means one bad plug, the engine can normally start with minimal symptoms. For that, it’s most common symptom would be the illumination of the Check Engine light. For vehicles under the freezing condition, 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
There can be multiple of factors to contribute this error code, such as:
- Faulty cylinder #9 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.
Nest, 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 #9 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 #9 glow plug
- Replacing the glow plug relay
Note that, if a P0670 trouble code is present along with this DTC, it is important to diagnose P0670 before diagnosing this code.