Error code

OBD2 P0684

OBD2P0684
Diagnostic Trouble Code· Standard OBD-II fault code read from the vehicle's onboard diagnostics.

What it means

Glow Plug Control Module to PCM Communication Circuit Range/Performance.

Severity

highThe vehicle may be drivable but with significant starting issues, especially when cold.

Summary

P0684 indicates an issue with communication between the glow plug control module and the PCM (Powertrain Control Module) on diesel engines, specifically related to Bank 2 of cylinders.

Symptoms

  • No start condition when the engine is cold
  • Extended cranking times required to start a cold engine
  • Erratic or fluctuating idle speed until the engine warms up
  • Thick clouds of white, grey, or black smoke during startup

Common Causes

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the glow plug control system
  • Low battery voltage(s)
  • Use of substandard aftermarket glow plug control modules
  • Defective glow plugs
  • Blown fuses and/or fusible links in the glow plug control system
  • Defective engine coolant sensors or other associated engine sensors
  • Defective PCM drivers for glow plug control
  • Failed or failing PCM (rare)

Diagnosis

  1. Check and repair any damaged wiring or connectors in the glow plug control system.
  2. Inspect for low battery voltage(s) and address if necessary.
  3. Replace substandard aftermarket parts with OEM equivalents.
  4. Test individual glow plugs for defects and replace as needed.
  5. Check fuses and fusible links in the glow plug control system and replace if blown.
  6. Verify engine coolant sensors and other associated sensors are functioning correctly.
  7. Scan PCM drivers for any faults related to glow plug control.

Fixes

FAQ

What is the most common cause of P0684?

The most common cause is damaged, burnt, shorted, disconnected, or corroded wiring and connectors in the glow plug control system.

Can I drive with this code?

You can potentially drive but may experience significant starting issues when cold. It's recommended to address the issue promptly.

View source

Not affiliated with OBD2. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.

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