P0684
UniversalGlow Plug Control Module to PCM Communication Circuit Range/Performance
Medium SeverityThe P0684 code indicates an issue with the communication circuit between the Glow Plug Control Module (GPCM) and the Powertrain Control Module (PCM). This communication failure can prevent the glow plugs from operating correctly, leading to difficult or failed engine starts, particularly in cold weather conditions, for diesel vehicles.
⚠️ Before You Begin (Safety First)
Always disconnect the battery before performing any electrical work on your vehicle. Allow the engine and surrounding components to cool completely before touching them to avoid burns. Wear appropriate personal protective equipment, such as safety glasses and gloves, during any repair process.
Common Symptoms
- Check Engine Light illuminated
- Hard starting, especially in cold weather
- Engine cranks but does not start
- Rough idle immediately after starting
- Increased engine warm-up time
Common Causes
- Faulty Glow Plug Control Module (GPCM)
- Damaged, corroded, or open wiring harness or connectors in the GPCM to PCM communication circuit
- A short circuit in the GPCM communication wiring
- Faulty Powertrain Control Module (PCM) (less common)
Frequently reported on: Ford F-Series Diesel (2003-2010), GM Duramax Diesel (2001-2010), Dodge Ram Cummins Diesel (2003-2010), Mercedes-Benz Sprinter Diesel (2007-2014)
Diagnostic Steps
- Check for any other related glow plug or engine control codes present, as they might provide additional diagnostic clues.
- Perform a thorough visual inspection of the wiring harness and connectors between the GPCM and the PCM for any signs of damage, corrosion, or loose connections.
- Using a multimeter and the vehicle's wiring diagram, check for proper voltage supply, ground, and continuity within the GPCM communication circuit.
- If all wiring and connections appear sound, consider testing or replacing the Glow Plug Control Module (GPCM).
- Clear the code and test drive the vehicle to see if the P0684 returns.
👨🔧 When to Call a Professional
If you lack experience with advanced electrical diagnostics, or if after checking basic wiring and replacing the Glow Plug Control Module the issue persists, it is strongly advised to consult a professional mechanic. Diagnosing intermittent communication issues or potential PCM faults requires specialized diagnostic tools and expertise.
Commonly Asked Questions
What is the Glow Plug Control Module?
The Glow Plug Control Module (GPCM) is an electronic unit in diesel vehicles responsible for managing the operation of the glow plugs, which preheat the combustion chambers to aid in cold engine starting.
Can I drive with a P0684 code?
While your vehicle might still run, especially in warmer climates, it is not advisable to ignore a P0684. You may experience significant difficulty starting, particularly in cold weather, and prolonged issues could potentially lead to other engine problems.
How do I test the GPCM?
Testing the GPCM typically involves checking its power and ground supply, as well as verifying communication signals with the PCM using a diagnostic scanner or oscilloscope. A detailed wiring diagram for your specific vehicle is essential.
Is P0684 related to bad glow plugs?
P0684 specifically indicates a communication problem between the GPCM and PCM, not necessarily a fault with the glow plugs themselves. However, a malfunctioning GPCM will prevent the glow plugs from operating correctly.
What is the most common cause of P0684?
The most common causes are a faulty Glow Plug Control Module or issues with the wiring harness and connectors within its communication circuit. A faulty PCM is a less frequent cause.