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P0684 OBD-II Trouble Code: Glow Plug Control Module to PCM Communication Circuit Range/Performance

check engine light

P0684 code definition

The P0684 OBD-II Trouble Code indicates that the glow plug control module to PCM communication circuit has a problem in the range/performance area of communications.

What the P0684 code means

The P0684 code means that there is a fault within the powertrain control module, the PCM, which will not allow it to communicate and send commands to the glow plug control module. This code does not determine the part of the system that has the problem. It merely indicates that there is a failure somewhere in that system. In some cases, the P0670 trouble code can occur along with this cost. This indicates that there is an issue with the wiring harness from the GPCM to the solenoid.

What causes the P0684 code?

A number of different things can cause the P0684 code to appear. The common causes associated with this trouble code include the following:

  • Failed glow plug
  • Failed GCPM
  • Loose connections
  • Corroded connections
  • Short in the wiring, such as from the PCM to the GPCM, the bus bar, or from the glow plug to the bus bar
  • Solenoid failure in the glow plug
  • Not enough battery power to solenoid

What are the symptoms of the P0684 code?

If your vehicle has a P0684 Code showing, then you may notice certain symptoms affecting your vehicle. You will notice that your Check Engine Light is on and the code is set. If there are only one or two failed glow plugs, then you might not notice many changes in the way the vehicle performs. However, if the engine is cold, it might be difficult to start. It could also have a miss until it has warmed up enough. If more than two glow plugs are affected by the issue, it will be extremely difficult to start the engine.

How does a mechanic diagnose the P0684 code?

The mechanic will utilize a tool called an OBD-II scanner. This tool connects to the vehicle and the Onboard Diagnostics II system they have on board, and provides help in diagnosing the vehicle, as well as data needed to determine the fault with the vehicle and the steps needed for repair. The technician will check the data, reset the code, and then test the vehicle to see if the code returns. If it doesn’t return, it is likely a false alarm. If it does return, it indicates to the mechanic that he or she needs to perform repairs.

How serious is the P0684 code?

The seriousness of the code will truly depend on the number of glow plugs that have failed. If it is only one or two, the vehicle should still start. More than that, and you will have real trouble. Regardless, when this trouble code appears, you should have a mechanic look at your vehicle.

What repairs can fix the P0684 code?

When a trained mechanic is trying to repair your vehicle and fix the P0684 code, they will first want to determine the true nature of the problem.

  • The mechanic will use the OBD-II scanner to check the code.
  • If the code returns after resetting, they will then inspect wiring and look for loose or corroded connectors.
  • They may inspect the glow plug bus bar and connections.
  • They will check the resistance between the wiring that runs from the glow plug to the bus bar.
  • They may also need to replace the GPCM if there is no power going to it.

Additional comments for consideration regarding the P0684 code

As a vehicle ages, it will often have sensor issues, some of which are false alarms and some of which are of legitimate concern. The mechanic’s testing can determine whether you have a real problem or not, so it’s not advisable to skip a visit to the mechanic even if you believe the code is not a real concern.

Need help with a P0684 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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