P0684 — Glow Plug Control Module to PCM Communication Circuit #

The definition of code P0684 may vary depending on the vehicle manufacturer (OEM). Always consult your vehicle-specific documentation for precise terminology.

• Glow Plug Control Module to PCM Communication Circuit Range/Performance (Ford, GM, Chrysler)
• Glow Plug Control Module Communication Fault (Volkswagen/Audi)
• Glow Plug Relay/Module Communication Circuit (Mercedes-Benz)

Summary of code P0684 #

P0684 indicates a problem in the communication circuit between the Glow Plug Control Module (the device that manages the pre-heating plugs essential for diesel engine starting) and the Powertrain Control Module (PCM), which is the main engine management computer. This code is set when the PCM detects a fault or loss of signal from the glow plug control system. Most commonly found on diesel engines, this issue can cause starting difficulties, especially in cold weather. The communication circuit acts as the digital handshake between these modules—if it falters, proper glow plug operation is compromised. Prompt attention is crucial to avoid prolonged starting issues and potential engine performance problems.

Common symptoms for trouble code P0684 #

If P0684 is present, your vehicle will likely exhibit one or more of the following symptoms. The severity can vary based on engine temperature and ambient conditions. In most cases, the symptoms are most apparent during cold starts:

• Hard starting, especially in cold weather
• Extended engine cranking time
• Rough idle after starting
• Glow plug warning light illuminated on the dashboard
• Possible reduction in engine power or limp mode
• Check Engine Light (CEL) illuminated

Probable causes for P0684 code (in order of frequency) #

Several potential issues can trigger code P0684. The most likely causes, ranked from most to least common, are:

1. Faulty Glow Plug Control Module — Internal failure or corrosion can disrupt communication with the PCM.
2. Wiring or connector issues — Damaged, loose, or corroded wires/connectors between the glow plug control module and PCM.
3. Faulty or corroded ground connections — Poor grounding can impede reliable signal exchange.
4. PCM malfunction — Rare, but possible if the PCM fails to send or receive messages correctly.
5. Blown fuse or relay related to the glow plug system — Loss of power to the circuit can mimic a communication fault.

Step-by-step diagnostic procedure for P0684 #

Accurately diagnosing P0684 requires a methodical approach to rule out basic faults before suspecting complex module failure. Here are the recommended steps:

• Scan for additional codes — Check for related DTCs (especially glow plug, PCM, or communication faults) to gather more context.
• Visually inspect wiring and connectors — Look for obvious damage, corrosion, or loose connections between the glow plug module and PCM.
• Test power and ground circuits — Verify the module receives proper battery voltage and has solid ground connections.
• Check fuses and relays — Inspect all glow plug and engine management fuses/relays for correct operation.
• Perform continuity and resistance testing — Use a multimeter to check for open circuits, high resistance, or shorts in the module-to-PCM wiring.
• Module communication test — With a scan tool, monitor communications from the glow plug module; compare data against OEM specs.
• If all else passes, test or replace the glow plug control module — Substitute with a known-good unit if available, or follow OEM test procedures.

Possible repairs for P0684 #

Once the root cause is identified, repairs can be targeted and effective. Typical solutions, matched to the probable causes, include:

• Repair or replace damaged wiring/connectors — Splice, solder, or replace as needed to restore proper connection.
• Clean or repair ground connections — Remove corrosion, tighten fasteners, and apply dielectric grease if appropriate.
• Replace the glow plug control module — If confirmed faulty, install a new OEM or quality aftermarket unit.
• Replace blown fuse or faulty relay — Always investigate why the fuse/relay failed before simply replacing.
• Replace or reprogram the PCM — Only after all other possibilities are exhausted and with professional confirmation.

Is it safe to drive with code P0684? #

Limited — While the vehicle may still operate, driving with P0684 can lead to hard starts, especially in cold weather, and may cause excessive wear on the starter and battery. Extended use without repair could also affect emissions and engine performance. For diesel engines, prompt repair is advised to avoid being stranded or causing further damage, particularly during winter.

Vehicles most commonly affected by P0684 #

Although P0684 can occur in any diesel vehicle with electronically controlled glow plug systems, it is most frequently reported in:

• Ford Super Duty (F-250, F-350, F-450) Power Stroke diesels
• Chevrolet/GMC Silverado and Sierra Duramax diesels
• Dodge/Ram Cummins diesels
• Volkswagen TDI models (Jetta, Passat, Golf, Touareg)
• Mercedes-Benz Sprinter and Bluetec diesel models

Common mistakes & best practices for P0684 #

Technicians and DIYers sometimes overlook key steps when addressing P0684. Here’s how to avoid common pitfalls:

• Skipping a visual inspection — Always check wiring and connectors before replacing major components.
• Replacing the glow plug module prematurely — Confirm all circuit and power/ground checks before suspecting the module.
• Ignoring related codes — Address all present codes for a complete repair.
• Not verifying repair — Clear codes and re-test to confirm the issue is resolved.
• Neglecting battery health — Weak batteries can mimic or exacerbate glow plug system faults.

Quick FAQ for code P0684 #

Q: Will a faulty glow plug control module always trigger P0684?
A: Not always. P0684 specifically points to a communication fault between the module and PCM. The module can fail in other ways that trigger different codes, or no code at all if the fault is internal and not detected by the PCM.

Q: Can I clear the code and ignore it if the engine starts fine?
A: Clearing the code may temporarily turn off the warning light, but if the underlying issue isn’t fixed, the code will return. Ignoring it can lead to increasingly difficult starts and eventual no-start conditions, especially in cold weather.