Possible causes

– Faulty Glow Plug Control Module (GPCM)
– GPCM harness is open or shorted
– GPCM circuit poor electrical connection
– Faulty Engine Control Module (ECM)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Lack/Loss of Power
– The engine may be harder to start
– Engine hesitation

P0684 Information for specific makes

P0684 VOLKSWAGEN