P0671

P0671 is a per-cylinder glow plug circuit fault for cylinder 1, indicating the module detected an open, short, or out-of-range resistance on that specific glow plug branch. Cheapest first: pull the valve cover (or remove the intake on top-of-head designs) and ohm-check the glow plug at the plug itself with the harness disconnected; a healthy glow plug reads roughly 0.5 to 2.0 ohms cold (varies by manufacturer, but values above 3 ohms or open circuit are bad, and values below 0.3 ohms indicate an internal short). Wiggle-test the harness connector at the plug since the rubber boots harden and crack from underhood heat. Caveat: do not replace all glow plugs as a set on a P0671 unless your testing confirms multiple plugs are weak, because the labor on stuck or seized glow plugs (especially on 6.0L Powerstroke and Mercedes OM642) can balloon a $30 part into a $2,000 cylinder-head-off repair if a plug breaks off in the head.

Vehicle-specific patterns: 2003 to 2007 Ford 6.0L Powerstroke routinely seizes glow plugs in the head, and P0671 on cylinder 1 (front driver side) is often paired with a snapped plug tip when extraction is attempted; this is the job that turns into a head-out repair if the tip drops into the cylinder. 2007 to 2013 Mercedes Sprinter and 2007 to 2009 Jeep Grand Cherokee with the OM642 3.0L V6 diesel are notorious for glow plug seizure and require Beru extractor kits. 2004 to 2010 Chevy and GMC 6.6L Duramax LLY/LBZ/LMM trips P0671 when a single glow plug shorts internally or the pigtail fails at the connector boot. 2009 to 2014 VW and Audi 2.0L TDI common-rail engines set P0671 from the dual-coil ceramic glow plugs failing one at a time. Estimated repair: $80 to $1,800.