Let me break down what code P0684 actually means for your vehicle. This code pops up when there’s a problem with the communication circuit between the Powertrain Control Module (PCM) and the Glow Plug Control Module (GPCM). In plain English, your car’s computer is trying to send signals to the system that controls the glow plugs, but something’s blocking or disrupting that message. Glow plugs are crucial for starting diesel engines, especially when it’s cold out-they heat up the combustion chamber so the engine can fire up smoothly. If this circuit isn’t working right, the PCM can’t properly control the glow plugs, which can make cold starts a real headache. This system is pretty straightforward, but it’s vital for reliable starting and smooth running when your diesel engine is cold.
DTC P0684
4762
6 minutes of readingCauses of P0684 code
From what I’ve seen in the shop and what’s confirmed in the OEM manuals, the most common reasons for a p0684, including dtc p0684 volkswagen, are usually electrical. Here’s what tends to go wrong most often:
- Broken or shorted wiring between the PCM, GPCM, glow plug bus, or the glow plugs themselves.
- Faulty glow plug (they do burn out over time, especially in cold climates).
- Loose or corroded connectors-these are notorious for causing intermittent issues.
- Failure of the Glow Plug Control Module (GPCM), though this is less common.
- Corroded or weak connections at the glow plug solenoid.
- Glow plug solenoid failure (rare, but it happens).
- Low battery voltage, which can prevent the solenoid from working right.
In my experience, wiring and connection issues are the usual suspects, so I always start there before moving on to the more expensive parts.
Symptoms of P0684
Here’s what you’re likely to notice if your vehicle sets a P0684 code. First off, the check engine light will come on. If only one or two glow plugs are affected, you might not notice much else-maybe just a slightly rougher start when it’s cold. But if more than two glow plugs aren’t working, starting the engine can get tough, especially on chilly mornings. Sometimes, the engine might misfire or run rough until it warms up. Basically, the colder it is and the more plugs that are out, the harder it’ll be to get going.
Diagnosis process for obd2 code P0684
Here’s how I usually tackle a P0684 code, step by step. I always recommend starting with the basics before diving into anything complicated:
- First, visually inspect all the wiring from the PCM to the GPCM, then to the glow plug solenoid, the bus bar, and finally to each glow plug. Look for any obvious signs of damage, loose connections, or corrosion.
- Disconnect the electrical connectors at the GPCM (usually black and green plugs). Check for bent pins, corrosion, or anything that looks out of place.
- With a multimeter, check each terminal for shorts to ground. If you find a short, repair it before moving forward.
- Apply dielectric grease to the connector pins and reconnect everything securely.
- Check the battery voltage and make sure all power and ground connections at the GPCM and solenoid are clean and tight. Weak battery or poor connections can cause all sorts of headaches here.
- Inspect the glow plug bus bar and make sure each wire is firmly attached and free of corrosion.
- Remove the wire from each glow plug and check for shorts to ground.
- Use an ohmmeter to test each glow plug. Place one lead on the glow plug terminal and the other on a good ground. A healthy glow plug should read between 0.5 and 2.0 ohms. Anything outside that range means the plug is bad.
- Check the resistance of the wiring from the glow plug to the bus bar-it should also be between 0.5 and 2.0 ohms. Replace any wires that don’t meet this spec.
- If all the above checks out and you’re still getting the code, consult the service manual for the specific wiring diagram. Double-check power and ground at the GPCM and solenoid. If there’s no power at the GPCM, the PCM may be at fault. If there’s power at the GPCM but not at the solenoid, the GPCM itself may need replacing.
It’s best to have someone assist you with some of these checks, especially when testing voltage and resistance.
Common mistakes with P0684 engine code
Honestly, I see a lot of folks jump straight to replacing glow plugs or the control module without checking the wiring and connectors first. That’s a surefire way to waste money and time. Another common mistake is not testing each glow plug individually-sometimes only one is bad, but people replace them all or miss the real culprit. Skipping the battery and ground checks is another big one; weak voltage can mimic a lot of these symptoms. Always start simple and work methodically, or you’ll end up chasing your tail.
Seriousness of dtc P0684
This isn’t something you want to ignore, especially if you rely on your diesel in cold weather. If the glow plug system isn’t working, you could end up stranded on a cold morning, or worse, damage your starter or battery from repeated hard starts. Over time, unburned fuel from poor cold starts can foul injectors or even cause cylinder wash, which is a serious hazard for your engine’s health. Don’t put this off-things can go south quickly if you let it slide.
Repair steps for P0684 trouble code
Here’s what usually needs to be done to fix a P0684 code, based on what’s worked in the field and what the manuals say:
- Repair or replace any damaged or corroded wiring and connectors between the PCM, GPCM, solenoid, bus bar, and glow plugs.
- Replace any glow plugs that test out of spec (not between 0.5 and 2.0 ohms).
- Clean and tighten all power and ground connections, especially at the battery, GPCM, and solenoid.
- If the GPCM isn’t sending power when it should, replace the module.
- If the solenoid is faulty (rare), replace it.
- Make sure the battery is fully charged and in good condition-sometimes that’s all it takes.
After repairs, always clear the code and test the system to make sure everything’s working as it should.
Conclusion
To sum it up, P0684 means your diesel’s glow plug control system isn’t communicating properly, which can make cold starts tough and lead to bigger problems if ignored. The issue is usually with wiring or connectors, so start there before replacing expensive parts. This is a moderate to high priority-don’t delay, especially if you’re heading into cold weather. The most reliable fix is a careful, step-by-step check of the wiring, connectors, and glow plugs, followed by targeted repairs. Take care of it promptly and you’ll avoid a lot of headaches down the road.
