P0031

P0031 means the PCM is seeing the heater control circuit pulled low — either the ground-side driver is shorted to ground, the heater element itself is shorted internally, or the supply voltage has collapsed. The PCM expects to see voltage rise on the control pin when it commands the driver off; if it stays near zero, the code sets. Cheapest-first: disconnect the sensor and ohm the heater pins at the sensor — narrowband should read 2-10 ohms cold, wideband 1-3 ohms cold. A reading below 1 ohm on a narrowband or below 0.5 ohm on a wideband indicates an internally shorted heater. With the sensor disconnected, key-on, the harness-side control pin should float near 12V on most platforms; if it sits at zero with the sensor unplugged, the harness is shorted to ground between the connector and the PCM. Verify the supply-side fuse and confirm 12V at the B+ pin key-on. Scope the ground-side: duty cycle pinned at 100% with low voltage confirms a wiring short rather than a driver failure. The expensive misdiagnosis is condemning the PCM for a driver fault when the real cause is a melted harness against the exhaust — always unplug and re-test the harness in isolation before any module work.

Vehicle-specific patterns: GM 5.3L LS trucks (Silverado/Tahoe/Yukon 2007-2014) routinely set P0031 when the upstream harness melts onto the manifold and shorts heater control to ground — covered under GM TSB PIT5358. Toyota 2GR-FE (Camry/Avalon/Sienna 2007-2012) wideband AFR sensors fail internally with a shorted heater, drawing excess current; Denso 234-9051 replacement is the fix. Subaru EJ25 (Forester/Outback/Legacy 2005-2012) front O2 sensors short internally with age, often after a misfire event cooks the element. Nissan VQ35DE (Altima/Maxima/Murano 2002-2008) sees harness chafe at the bracket near the bellhousing causing intermittent P0031. Estimated repair: $120 to $400.