Damaged or corroded wiring or connectors between the glow plug control module and the PCM; Faulty glow plug control module, often due to internal failure or water intrusion; Poor electrical connections at the PCM or glow plug control module, such as loose pins or grounds; Open or shorted communication circuit, including chafed wires or broken conductors; Low battery voltage or charging system issues causing erratic module communication; Failed PCM, though this is less common and should be diagnosed last

DTC codes / P06xx · Computer and output circuits / P0684

P0684Glow Plug Control Module to PCM Communication Circuit Range/Performance

P0684 indicates that the powertrain control module (PCM) has detected a communication problem with the glow plug control module, specifically that the signal circuit is outside the expected range or performance criteria. The PCM monitors the dedicated communication line between itself and the glow plug control module, and when the voltage or timing of the signal does not match what the PCM expects, it sets this code. This is a generic code that applies to all OBD-II vehicles with a glow plug system, typically diesel engines.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illumination of the check engine light or service engine soon light
Hard starting or extended cranking in cold weather due to glow plugs not functioning
Rough idle or white smoke after a cold start because glow plugs are not heating properly
Often no symptoms besides the warning light, especially in warmer weather when glow plugs are less critical
Possible reduced engine performance or limp mode if the PCM limits operation to protect the system
In some vehicles, a glow plug indicator light may flash or stay on continuously

Common causes, most likely first

Damaged or corroded wiring or connectors between the glow plug control module and the PCM
Faulty glow plug control module, often due to internal failure or water intrusion
Poor electrical connections at the PCM or glow plug control module, such as loose pins or grounds
Open or shorted communication circuit, including chafed wires or broken conductors
Low battery voltage or charging system issues causing erratic module communication
Failed PCM, though this is less common and should be diagnosed last

How to diagnose and fix it

Perform a visual inspection of the wiring harness and connectors between the glow plug control module and PCM for damage, corrosion, or loose pins
Check battery voltage and charging system operation to ensure stable power supply to both modules
Use a scan tool to retrieve all stored codes and freeze frame data, and check for related communication codes
Test the communication circuit for continuity, short to ground, and short to power using a digital multimeter
Verify power and ground supplies to the glow plug control module and repair any faults
If the circuit tests good, substitute a known-good glow plug control module and clear the code to see if it returns
If the code persists after module replacement, diagnose the PCM and its connections, and reprogram or replace as needed

Don't Replace the Glow Plug Module Yet

A common oversight is assuming the glow plug control module is defective when the real problem is a chafed wire in the harness that only shorts intermittently when the engine moves or heats up.
This code can be set by a weak battery or poor ground connection that causes voltage spikes or drops, so always load-test the battery and inspect ground straps before condemning any module.
On many vehicles, the glow plug control module communicates with the PCM over a dedicated serial data line, and a failure in that line can also trigger other communication codes that may distract from the root cause.
Replacing the glow plug control module without clearing adaptive learning values or performing a relearn procedure can cause the code to reappear immediately, so follow the manufacturer's reset steps.
P0684 is sometimes stored alongside glow plug circuit codes for individual cylinders, but those codes indicate faulty glow plugs or wiring to the plugs, not the communication line itself.

Can I keep driving with P0684?

You can generally drive the vehicle with this code, but cold starting may be difficult and extended cranking can wear the starter and battery. Avoid relying on the vehicle in freezing temperatures until the fault is repaired, and address the issue promptly to prevent further damage to the glow plug system.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.