What causes a P0684 code?

P0684 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a P0684 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P0684 active.

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

OBD-II

P0684

Glow Plug Control Module to PCM Communication Circuit Range/Performance

What it means

This fault indicates a communication or circuit integrity issue between the Glow Plug Control Module (GPCM) and the Powertrain Control Module (PCM), specifically regarding the GPCM feedback signal.