P2270

P2270 means the post-catalyst O2 sensor on Bank 1 (downstream sensor 2) is reporting a fixed lean voltage below roughly 0.2 V and not switching across a forced fuel enrichment, which is the ECM's primary canary for catalyst efficiency loss or sensor failure. Cheapest-first ladder: graph upstream and downstream O2 voltages together under closed-loop cruise - a healthy downstream sensor should hover at 0.6 to 0.8 V with minimal switching while the upstream switches 0.1 to 0.9 V at 1+ Hz; if the downstream is pinned below 0.2 V it is either stuck, unheated, or seeing an exhaust leak ahead of it. Caveat: on diesels, P2270 frequently sets after a DPF service when techs forget to torque the V-band clamp at the cat outlet, so always pressure-test the exhaust with a smoke machine before condemning the sensor.

Vehicle-specific patterns: 2011 to 2016 Duramax LML 2500HD set P2270 from a cracked NOx sensor 2 housing post-SCR, GM PI0958; 2011 to 2019 Ford 6.7L Powerstroke set the code when the downstream O2 heater circuit fails; 2007 to 2009 Dodge 6.7L Cummins set P2270 from exhaust leaks at the DPF inlet V-band; 2010 to 2014 VW/Audi 2.0 TDI Common Rail set P2270 from carbon-fouled downstream sensors. Estimated repair: $80 to $620.