What Does P0684 Mean?
The PCM has detected that the communication between the glow plug control module (GPCM) and PCM is outside its expected range or performance parameters. The communication link exists but is transmitting data that is erratic, delayed, or inconsistent. This can cause incorrect glow plug timing, improper glow duration, or false glow plug status reporting to the driver.
Common Causes
30%
Intermittent connection in the GPCM-to-PCM communication circuit
30%
Glow plug control module with degraded communication output
20%
High resistance in communication wiring due to corrosion
10%
Low voltage supply to GPCM affecting signal quality
10%
PCM communication port marginal performance
Diagnostic Steps
1
Monitor the GPCM communication data stream in real time using the scan tool; look for erratic values, dropouts, or data that doesn't match actual glow plug behavior.
2
Wiggle test the GPCM and PCM connectors while monitoring communication data for intermittent disruptions that indicate a loose connection.
3
Measure the voltage supply and ground at the GPCM; ensure battery voltage is present at the power pin and ground resistance is less than 0.1 ohms.
4
Check the communication wiring for proper resistance values and verify no excessive resistance exists at any splice or connector junction.
5
Monitor the communication signal with an oscilloscope to verify proper signal amplitude and timing; degraded signals indicate a hardware failure in the GPCM or wiring.
Estimated Repair Cost
$150 - $800
Parts + labor, varies by vehicle and location
The PCM has detected that the communication between the glow plug control module (GPCM) and PCM is outside its expected range or performance parameters. The communication link exists but is transmitting data that is erratic, delayed, or inconsistent. This can cause incorrect glow plug timing, improp...
The most common cause of P0684 (Glow Plug Control Module to PCM Communication Circuit Range/Performance) is: Intermittent connection in the GPCM-to-PCM communication circuit
Typical repair costs for P0684 range from $150 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Generally safe to drive. The glow plug system may operate with degraded timing or accuracy, leading to suboptimal cold starts. The dash glow plug indicator may not display correctly. Repair before cold weather to ensure reliable starting.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0684 to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Cold Start / Glow Plug System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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