P0684 Fault Code

Long Description

Fault code P0684 is logged when the vehicle’s primary computer, the powertrain control module (PCM), has detected an issue with the signal between its communication circuit and the GPCM (GPCM).

This issue occurs when the PCM detects a signal that is outside of the allowable range within the communication circuit to the GPCM. These can also be signals that the PCM has deemed as abnormal.

This can be logged alongside P0683.

Unlike petrol engines, diesel engines do not rely on spark plugs for ignition. Instead, they use the heat generated from the compression cycle to cause combustion.

At low engine temperatures and cold environments, there is not enough heat generated in the beginning to ignite the air-fuel mixture. Glow plugs warm the cylinders to aid in the first few cycles when starting the engine.

This fault code is only possible in vehicles with diesel engines. Manufacturers like Audi, Volkswagen, and Ford are some of the most commonly associated with this fault code.

Other Signs of This Problem

Issues involving glow plugs can cause similar symptoms to issues with spark plugs. However, these are typically relegated to the starting of the engine rather than during continuous running.

The most obvious sign of this type of issue is that the Check Engine light will be illuminated. It is also possible for the Pre-Glow light will stay illuminated when starting the car or not illuminate at all.

Other issues common to glow plug malfunctions like no or hard starts can also occur. This is particularly true for engines that are cold or vehicles in cold environments.

Other glow plug fault codes could also help determine the source of this issue. These can be P0670 up to P0682.

What Could Be The Problem?

The causes for this issue are likely found within the circuit linking the GPCM and the PCM. some components in the glow plug system can also be the cause.

A faulty GPCM can be one of the most likely causes of this P0684 issue.

Other electrical faults can also lead to this code being logged. These can be things like damaged wires, loose or corroded connections, or blown fuses.

A defective glow plug can also cause this erroneous signal to be sent.

A glow plug solenoid failure is another possible cause. This can also occur as a result of insufficient power being sent to the solenoid.

Fixes

Fixes for this type of issue are typically electrical in nature, focusing on the wiring harness between the PCM and GPCM. However, some parts may need to be replaced.

The wiring between the two control modules should be checked for faults. These need to be repaired. Pay special attention to loose connections and frayed wires. Corrosion should also be cleaned.

The wiring for the glow plug bus bar also needs to be checked for shorts or damage.

Any faulty glow plugs should be replaced.

Seriousness

Fault code P0684 and the other glow plug codes are moderately serious issues. The engine can be damaged over time from continuously starting with a faulty glow plug.

Depending on the severity of the issue, it may become difficult to start the vehicle.

It is a good idea to have this issue fixed immediately if possible as this can hide more issues with other glow plugs. This is especially true for cold climates.