P0684 Trouble Code: Meaning, Symptoms, Causes & Fixes

If you’ve stumbled upon this guide, chances are your car’s dashboard is flashing the P0684 trouble code, and you’re on a mission to figure out what it means and how to fix it.

You’re in the right place. We’ll cover everything from symptoms to solutions.

Having wrangled with the P0684 code in my garage more than a few times, I assure you, I’ve got the inside scoop on getting your car back in tip-top shape.

In this post, we’ll specifically zero in on the P0684 code, which deals with the glow plug control circuit. We’ll decode its meaning, unravel the symptoms, explore the causes, and, most importantly, guide you through the fix process.

So, buckle up and let’s get rolling on this automotive journey.

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What Does P0684 Trouble Code Mean?

When your car coughs up a P0684 trouble code, it’s signaling trouble in paradise between the glow plug control module and the PCM.

But let’s break it down barney-style, without getting lost in the technical jungle.

First off, glow plugs are the unsung heroes for diesel engines, especially on cold mornings. They warm up the engine, ensuring it starts smoothly.

The control module is the brains of the operation, deciding when and how these plugs do their magic. The PCM? That’s the big boss overseeing your car’s electronic systems.

Now, imagine the control module trying to chat with the PCM about when to fire up those glow plugs, but the message gets garbled or lost.

That’s your P0684 in a nutshell. It’s like sending a text message that never delivers.

The result?

Your diesel engine might act like it’s not a morning person, showing reluctance to start or performing poorly in cold conditions.

In essence, this code is a red flag that there’s a hiccup in the conversation between these two critical components. It doesn’t say exactly why the chat is breaking down, but it lets you know it’s happening.

And in the automotive world, knowing is half the battle. Ignoring this warning could lead to rough starts or, worse, not starting at all when the mercury dips.

So, paying attention to this code is crucial for maintaining smooth operations and ensuring your diesel engine is ready to go when you are.

P0684 Trouble Code Symptoms

If your vehicle is showing the P0684 trouble code, you might notice several symptoms that indicate trouble with the glow plug control module communication. Here’s what to keep an eye out for:

• Difficulty Starting: The engine may struggle to start, especially in cold weather, as the glow plugs are not being activated properly.
• Engine Misfires: Poor communication can lead to untimed glow plug activation, causing the engine to misfire.
• Rough Idling: You might notice that the engine idles roughly when it’s cold, smoothing out as it warms up.
• Decreased Fuel Efficiency: If the glow plugs are not working efficiently, the engine will consume more fuel, especially during startup.
• Check Engine Light: The most obvious symptom that alerts you to an issue will be the check engine light coming on.
• Cold Start Issues: Particularly in diesel engines, failing to start on a cold morning might be a direct symptom of this trouble code.
• Error Codes From Scanner: Apart from P0684, additional diagnostic trouble codes related to the glow plug system might also be present.

P0684 Trouble Code Causes

Experiencing the P0684 trouble code can be due to a variety of issues within the communication line or components connected to the glow plug control module and the PCM. Here are the primary causes:

• Faulty Glow Plug Control Module: The most common culprit, a defective module won’t manage glow plug operation effectively.
• Damaged Glow Plugs: If the glow plugs themselves are damaged, they won’t respond correctly to the module’s commands.
• Poor Wiring Connections: Bad or corroded wires and connectors between the PCM and the glow plug control module can disrupt communication.
• Faulty PCM: Although rare, a problem with the Powertrain Control Module itself can manifest as this trouble code.
• Software Needs Updating: Sometimes, the vehicle’s software may need an update to ensure smooth communication between the PCM and other modules.
• Ground Faults: Inadequate grounding can also interfere with the electronic signals that control the glow plugs.

How To Fix A P0684 Trouble Code

Tackling a P0684 trouble code requires a methodical approach to pinpoint the root cause and apply the right fix. Here’s how you can diagnose and repair the issue:

1. Diagnostic Scanner Check

Start with a diagnostic scanner to confirm the P0684 code and check for any other related codes that could give clues about the underlying issue.

2. Visual Inspection

Examine the wiring and connectors between the glow plug control module and the PCM for signs of damage, corrosion, or loose connections. Repair or replace any compromised components.

3. Glow Plug Control Module Testing

Use a multimeter to test the glow plug control module’s operation. A malfunctioning module will need to be replaced.

4. Glow Plug Inspection

Check each glow plug’s resistance with a multimeter. A significant deviation from the manufacturer’s specified resistance indicates a faulty glow plug that needs replacement.

5. PCM and Software Check

Ensure the PCM is functioning correctly and that the vehicle’s software is up to date. Sometimes, a simple software update can resolve the issue.

6. Ground Connection Verification

Inspect the ground connections for the PCM and glow plug control module. Clean and secure any loose or corroded ground points.

7. Circuit Testing

If the problem persists, conduct a detailed circuit test to identify any hidden issues like short circuits or breaks in the wiring that could be affecting communication.

Once the root cause is identified and rectified, clear the trouble codes with your diagnostic scanner, and take the vehicle for a test drive to ensure the issue has been resolved. Remember, addressing a P0684 code promptly can prevent more serious problems down the road, keeping your diesel engine starting smoothly and running efficiently.

Conclusion

You’ve now got the lowdown on what a P0684 trouble code means and the steps to fix it. With this guide, diagnosing and repairing this issue should be a breeze.

Have you encountered a P0684 code before? What was the root cause in your situation?

Feel free to drop a comment below! Share your experiences or ask questions about troubleshooting the P0684 code. Your insights could help fellow readers!