P0680

P0680 means the ECU detected a glow plug circuit fault on cylinder 9 — open, short, or out-of-spec resistance from the GPCM. The cheapest-first diagnostic ladder: (1) Ohm the cyl 9 plug at its terminal — healthy 0.4-2.0 ohms cold (most diesels 0.6-1.2 ohms), 0 shorted, infinite open, 4+ weak. (2) Verify GPCM PWM command at 0-100% duty cycle with glow-on time 5-30 sec scaled to coolant temp. (3) Amp probe on the cyl 9 feed should show 7-20A initial draw dropping to 5-15A as the plug heats; flat 0A with command ON = open, sustained 20A+ = short. Cylinder 9 only exists on 10-cylinder or 12-cylinder diesels, which in light-duty/medium-duty shop work means a V10 — the VW V10 Touareg TDI is the most common 10-cylinder diesel a North American shop will see, with the GM Detroit Diesel 6.5L V8 mapping cyl 9 to special GPCM channels in some industrial/marine configurations. Test resistance INDIVIDUALLY at the glow plug terminal — pulling a shorted plug measures 0 ohms; an open plug measures infinite; a marginally weak plug measures 4+ ohms cold and is the most common cold-start hard-start root cause. Replace as a set on diesel engines — mixed-age plugs cause progressive failure.

Vehicle-specific patterns: 2004-2010 VW Touareg V10 TDI (5.0L BWF/BLE) is the primary P0680 platform in North American shops — cylinder 9 is on the rear of the right bank and is one of the hardest plugs to reach on a chassis that already requires removing the front clip (the famous Touareg V10 service position) for any major engine work; expect 4-6 hr labor on a single plug. The V10 TDI's glow plug system uses a Beru GPCM that's known to fail in cold climates and throws P0680 + multiple other per-cyl codes simultaneously. 2001-2007 Audi A8 / Q7 V12 TDI in European-market shops throws P0680 from the same Beru/Bosch GPCM family. 1994-2002 GM Detroit Diesel 6.5L V8 in commercial/military Hummer applications maps cyl 9 to an extended GPCM channel in some configurations — verify the FSM matches the wiring before assuming a hardware fault. Estimated repair: $400 (single plug on a 6.5 industrial) to $4,500 (Touareg V10 single plug with required front-clip service access).