P0161

P0161 is a heater-circuit fault on bank 2 DOWNSTREAM (B2S2) — bank-2 mirror of P0141. Heater current outside the expected window. Ohm the heater pins (6-15Ω cold narrow-band, 2-4Ω wideband); open inside the sensor = replace. Verify B+ at heater feed pin and pulsed ground at the ECU control pin. Downstream heaters fail more often than upstream because of the harsher thermal cycling (sitting right downstream of a cat that hits 800°C+ then rapidly cooling on shutdown). Check the O2 heater fuse first — many platforms share one fuse across all four sensor heaters, so a single shorted heater on one corner can blow the fuse and set P0135 + P0141 + P0155 + P0161 simultaneously; replacing all four sensors when really one is shorted and the fuse blew is a classic parts-cannon mistake.

Vehicle-specific patterns: 2003-2008 Toyota Sequoia / Tundra 4.7L 2UZ-FE — longitudinal, bank 2 passenger side; throws P0161 around 140k miles. 2005-2010 Chrysler 300 / Charger 5.7L Hemi — TIPM heater-relay failure pattern hits downstream sensors too; same TSB territory as P0135/P0141/P0155. 2007-2013 Chevy Avalanche / Tahoe 5.3L AFM — longitudinal, bank 2 driver side; sets P0161 from corroded downstream connector under road-spray zone. 2009-2016 Subaru Outback / Forester FB25 — flat-4 with bank 2 on the driver side; throws P0161 from a chafed harness against rear differential mount (same root as P0141 on these). Cost band: $150-$350 for sensor swap; up to $700+ if TIPM is the real cause; under $25 if it's a shared O2 heater fuse.