P0684 - Glow Plug Control Module (GPCM) To Powertrain Control Module (Pcm) Communication Circuit Range/Performance

WARNING: This page is about a different car, the 2008 Ford Pickup, 2008 Ford F550 Super Duty, 2008 Ford F450 Super Duty, and 2008 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

P0684 - GLOW PLUG CONTROL MODULE (GPCM) TO POWERTRAIN CONTROL MODULE (PCM) COMMUNICATION CIRCUIT RANGE/PERFORMANCE

Description:	During the glow plug system check, the PCM enables the GPCM and monitors the GPCM/PCM communication circuit for a response. The DTC sets when the PCM stops receiving messages or receives an error message from the GPCM over the communication circuit.
Possible Causes:	GPCM/PCM communication circuit open in the harness GPCM/PCM communication circuit short to ground in the harness GPCM/PCM communication circuit short to voltage in the harness
Diagnostic Aids:	This DTC may be set as a result of the concern with GPCM control circuit. Diagnose DTC P0670 before diagnosing DTC P0684.
Application		Key On Engine Off	Key On Engine Running	Continuous Memory
All		refer to PINPOINT TEST AF .