P0036

P0036 covers the heater control circuit for the downstream O2 sensor on Bank 1 — the post-catalyst sensor used to monitor catalytic converter efficiency. It is almost always a narrowband sensor. The PCM monitors heater control activity the same way as upstream, and the sensor lives in a cooler exhaust location, so heater duty stays higher longer. Cheapest-first: ohm the heater cold at the sensor connector — expect 2-10 ohms narrowband, with most Bosch downstream units at 3-7 ohms and Denso at 11-14 ohms cold (Toyota downstream sensors are an exception with higher cold resistance). Verify supply voltage at the B+ pin key-on; the downstream heater frequently shares a fuse with multiple sensors, so a blown fuse will set P0036 plus P0030/P0050 simultaneously. Scope the ground-side PWM — closed-loop entry should occur within 30-90 seconds. Heater current 0.3-1.5A is normal for downstream sensors (lower than upstream because they're smaller). Expensive misdiagnosis: replacing the downstream sensor when the real fault is a shared fuse, a melted harness near the catalyst heat shield, or a corroded body-ground that the heater driver returns through.

Vehicle-specific patterns: GM 5.3L LS trucks (Silverado/Tahoe/Suburban 2003-2014) chronically melt the downstream harness against the catalyst heat shield — the same wiring failure pattern as upstream, just further back. Ford 5.4L Triton (F-150/Expedition 2004-2010) sees downstream connector corrosion from road-salt intrusion at the rear of the cat. Toyota 2GR-FE (Camry/Sienna 2007-2014) downstream Denso narrowbands age out around 150K with internal heater opens, typically Denso 234-4622. Chrysler 5.7L HEMI (Ram/300/Charger 2005-2014) suffers a shared O2 heater fuse (Fuse 17 in the TIPM on many years) that fails and throws P0036+P0056 together — replace the fuse and inspect the TIPM for the underlying short. Estimated repair: $90 to $400.