P0684Generic code

P0684 Code: Glow Plug Control Module to PCM Communication Circuit Range/Performance

The P0684 code means the information the glow plug control module sends to the computer is not consistent. Check the glow plugs and the module, because a plug with an odd resistance can confuse the module. It is a low severity code and you can keep driving.

Severity
Low
Can I keep driving?
Yes, no immediate risk
Estimated cost
$30 to $600
Difficulty
Medium
System
Ignition and Misfires
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0684 code means

The module sends the computer the status of the glow plugs. If that data does not make sense, the computer stores the P0684 code.

Check the glow plugs and the module. A glow plug with an abnormal resistance can confuse the module and throw the data out of range.

This code often shows up right after glow plugs were replaced with plugs of a different resistance, or from a different brand than the original. The module expects a specific current and reads anything else as a fault. Always use glow plugs that match the correct specification.

P0684 code symptoms

  • Hard starting when the engine is cold.

Likely causes of the P0684 code

  1. Glow plugs with an abnormal resistance. They confuse the diagnosis of the module.
  2. Glow plug module with damaged internal electronics.
  3. Wiring damaged by heat, rubbing or corrosion.

How to diagnose the P0684 code step by step

  1. Measure the resistance of the glow plugs and compare them with each other.
  2. Inspect the wiring of the module for heat damage, chafing and corrosion.
  3. Confirm the glow plugs are the correct specification. A different resistance confuses the module.

How to fix the P0684 code

  • Replace the glow plugs or the module, depending on what the tests show.

Technical reference data

  • Glow plug resistance: 0.5 to 2 ohms.
  • Reference values only: check the manufacturer data for your vehicle.

Brand-specific notes

P0684 on Ford

The P0684 code is common on the Ford Power Stroke.

Frequently asked questions about P0684

What does the P0684 code mean?
It means the data the glow plug module sends to the computer is not coherent. A glow plug with the wrong resistance, the module or the wiring can all cause it.
Can I drive with a P0684 code?
Yes. It is not a serious fault, and the engine runs normally once warm. The main trouble is a harder start on cold mornings.
What should I check first?
Start with the glow plugs and the module. Measure each plug and compare the readings, since one odd plug can throw off the whole diagnosis.
Is it serious?
No, unless the weather is cold. Then it matters more, because a glow plug system that is not working can make the engine very hard to start.
How much does it cost to fix P0684?
Between $30 and $600, depending on whether you need a set of glow plugs, a module or just a wiring repair.

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Questions about the P0684 code

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