Fault code P0679 – cylinder 9 glow plug circuit malfunction

Fault code P0679 is called “Cylinder 9 Glow Plug Circuit Malfunction” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0679

OBD-II trouble code P0679 is set when the PCM detects an electrical defect, which is defined as an open circuit in the glow plug control circuit of cylinder #9. This code is also set when the PCM detects a voltage change of 10% or more of the required voltage.

Fault code P0679 – cylinder 9 glow plug circuit malfunction

The glow plug is installed in the cylinder head very close to the point that initiates combustion, or “hot spot. When the engine control computer determines that the engine is cold using the oil and transmission sensors. It decides to help the engine start using glow plugs.

Starting too long will cause the glow plugs to melt because they produce heat due to high resistance and glow red hot when activated. This intense heat is quickly transferred to the cylinder head. Which allows the combustion heat to persist for the fraction of a second it takes to ignite the incoming fuel to start.

Code P0679 tells you that something in the glow plug circuit is malfunctioning, causing the glow plug in cylinder #9 not to heat up. The entire circuit must be checked to find the fault.

Symptoms of vehicle malfunctions

The main signal that an error P0679 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The Check Engine light on the control panel will illuminate (the code will be written to the ECM memory as a fault).
  2. The engine is difficult to start in cold weather or when it is idle long enough to cool the block.
  3. Lack of power until engine warms up sufficiently.
  4. Engine vibration and jerking when accelerating.
  5. Engine may be louder than usual when starting.
  6. Preheat indicator light does not turn off.
  7. White smoke may be visible at startup.

If only one glow plug fails, symptoms will be minimal because the engine usually starts with one faulty glow plug. But in cold conditions, you are more likely to experience the listed problems associated with fault P0679.

Factors that can cause this error code

The error code P0679 can mean that one or more of the following problems have occurred:

  • Defective glow plug of cylinder No. 9.
  • Open or short circuit in glow plug control circuit.
  • Plug connector is damaged.
  • Glow plug control module is defective.
  • Blown fuses may also be the cause.
  • Sometimes a faulty PCM module is the cause.

How to fix or reset OBD-2 code P0679

Some suggested steps for troubleshooting and fix the error code P0679:

  1. Read all stored data and error codes with an OBD-II scan tool.
  2. Clear the error codes from the computer memory.
  3. Test-drive the vehicle to see if error P0679 appears again.
  4. Visually inspect the wires and connectors for damage.
  5. Inspect the fuses, and replace the blown one if necessary.
  6. Check the glow plug.
  7. Test the engine control module (ECM).

Diagnose and repair of problems

Check the glow plug by disconnecting the connecting wire. Take a voltmeter and set it to the “ohm” measurement. Place the red wire on the glow plug terminal and the black wire on the ground.

In most cases, the range should be 0.5 to 2.0 ohms (check the data against the factory service manual). If outside the acceptable range, replace the glow plug.

Check the wire going to glow plug #9 by placing the red wire on the tire wire and the black wire on the side of the glow plug. The resistance should be the same, 0.5 to 2.0 ohms. Accordingly, if it is higher, replace the wire to the glow plug from the tire.

Check the wires for loose, cracked or missing insulation. Connect the code scanner to the OBD port under the dashboard and turn the key to the “on” position with the engine off. Clear the codes. Do a test drive to make sure the problem with error P0679 is gone.

On which vehicles does this problem occur most frequently

Fault code P0679 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Ford
  • Jeep
  • Opel

Fault code P0679 can sometimes be found with other errors. The most common are the following: P0670, P0671, P0672, P0673, P0674, P0675, P0676, P0677, P0678, P0680, P0681, P0682.


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