P0136

P0136 is a circuit fault on bank 1 DOWNSTREAM (B1S2) — the post-catalyst sensor. This is the cat-monitoring sensor, not a fuel-trim sensor; on a healthy car its signal sits relatively flat at ~0.6-0.8V (narrow-band) because the cat has consumed most of the free oxygen in the exhaust stream. P0136 specifically means the signal is out-of-range implausible or the ECU can't even bias the pin to its rest voltage — distinct from P0137 (stuck low) and P0138 (stuck high). Cheapest-first: pull the connector and inspect for the usual downstream culprits — exhaust soot/oil migration through a leaking gasket, road salt corrosion on northern cars (this sensor sits in the worst environment on the vehicle), and rodent damage to the harness where it routes near the floor pan. Backprobe the signal wire at the ECU and verify the ~0.45V bias is present key-on engine-off; if it's at 0V or B+, the harness is shorted. Note that a P0136 alone rarely means cat failure — that's P0420 territory. P0136 is sensor or wiring.

Vehicle-specific patterns: 2005-2010 Chevy Cobalt / HHR with the 2.2L Ecotec set P0136 from a known-defective sensor lot (GM TSB 06-06-04-026 addressed early failures). 2007-2013 Toyota Tundra / Sequoia 5.7L 3UR-FE throws P0136 when the downstream harness gets pinched between the transmission crossmember and the heat shield. 2003-2009 Nissan 350Z / G35 VQ35 sets P0136 from a chafed wire where the downstream harness crosses the rear subframe — common past 100k. 2008-2014 Dodge Grand Caravan / Town & Country 3.6L Pentastar throws P0136 after the rear (bank 1 on this transverse V6) heat shield rusts through and exposes the sensor pigtail. Cost band: $150-$350 for downstream sensor swap (cheaper than upstream because access is usually easier); under $80 if it's harness repair.