Damaged, corroded or loose wiring or connectors between the glow plug control module and the PCM; Low battery voltage or poor power and earth supply to the glow plug control module; Faulty glow plug control module with internal communication fault; Water ingress or heat damage at the module or its connector; Network interference or a fault on the vehicle communication bus affecting several modules; Outdated or faulty PCM software where a manufacturer update is known to address the fault; Faulty PCM communication driver.

P0684

Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
84: FaultThe specific fault in that area

What it means

P0684 is a generic powertrain code meaning the communication between the glow plug control module and the engine control unit (PCM) is outside the expected range or not performing as it should. The PCM has detected missing, implausible or corrupted messages on the link, or a mismatch between what it commands and the feedback it receives. Whether the link is a dedicated wire, a serial data line or a vehicle network depends on the make and engine.

Symptoms

Glow plug warning light on or flashing
Hard starting or extended cranking on a cold engine
White smoke and rough running after a cold start
Engine management warning light on
Increased exhaust smoke or poor idle until warm
Other communication or glow plug codes stored alongside

Possible causes

Damaged, corroded or loose wiring or connectors between the glow plug control module and the PCMlikely
Low battery voltage or poor power and earth supply to the glow plug control modulelikely
Faulty glow plug control module with internal communication faultpossible
Water ingress or heat damage at the module or its connectorpossible
Network interference or a fault on the vehicle communication bus affecting several modulespossible
Outdated or faulty PCM software where a manufacturer update is known to address the faultrare
Faulty PCM communication driverrare

How to diagnose

Read all stored and pending codes and note freeze frame data, including battery voltage and coolant temperature
Check for other communication, network or glow plug circuit codes that point to a wider problem
Test battery condition and charging voltage, as low voltage can upset module communication
Inspect the glow plug control module, its connector and the wiring for damage, corrosion, moisture and chafing
Check the module power supply, fuses and earth connections for voltage drop under load
Test the communication circuit between module and PCM for continuity, short circuits and resistance to the manufacturer specification
Use live data or an active test to see whether the module responds and reports glow plug status
Check for applicable software updates or technical bulletins for the specific model and engine

How to fix

Repair or replace damaged wiring and clean or replace corroded connectors
Charge or replace a weak battery and correct any charging or earth faults
Replace the glow plug control module if it is confirmed faulty, and code or adapt it if the vehicle requires
Seal or correct the cause of water ingress or heat damage at the module
Repair the underlying network fault if several modules are affected
Update the PCM calibration only where an outdated version is a known cause for that vehicle

Can I keep driving?

Medium severity

The car can usually be driven, but cold starting may be poor and smoke and emissions may increase. Get it checked soon, particularly before cold weather, as starting problems may get worse.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.