P2031

P2031 means the post-DPF exhaust gas temperature sensor (commonly EGT 12 or EGT 13 depending on layout) is reporting a voltage outside the ECM's expected 0.2 to 4.8 V window or has gone open circuit, which immediately disables active regen because the ECM cannot trust regen completion temperature. Cheapest-first ladder: unplug the sensor and back-probe the harness for 5 V reference and clean ground (under 0.1 V drop), then ohm the sensor cold (typical NTC EGT reads 200 to 400 kOhm at 20 C and drops to under 200 ohm at 600 C); if the harness is good and resistance is open or stuck, the sensor itself is the failure. Compare its live reading to the upstream EGT at idle on a cold start - the two should be within 30 C of each other and within 100 C of intake air temp before the engine warms. Caveat: the sensor wires run along the turbo heat shield on most platforms and chafe through, so always inspect the full harness run before condemning the sensor.

Vehicle-specific patterns: 2008 to 2010 Ford 6.4L Powerstroke chronically fail the EGT 13 (post-DPF) sensor from thermal cycling during regen, covered by Ford TSB 09-25-7; 2007.5 to 2012 Dodge Ram 6.7L Cummins fail the downstream EGT sensor when its pigtail melts against the DPF heat shield, fixed by Mopar TSB 11-002-13; 2011 to 2016 Duramax LML sets P2031 when the EGT 4 sensor (post-DPF) corrodes internally from condensation; 2010 to 2015 VW/Audi 3.0 TDI fail the post-DPF EGT from soot fouling. Estimated repair: $90 to $480.