P0138

P0138 says bank 1 downstream (B1S2) signal is stuck HIGH — over ~0.95V on a narrow-band, indicating a permanently-rich post-cat exhaust. Three possibilities, cheapest-first: the engine really IS dumping unburnt fuel through the cat (look for upstream rich codes P0172/P0132, a failed coolant-flooded cat, or an exhaust leak upstream of B1S1 that's making the ECU over-fuel); the sensor signal wire is shorted to its own heater B+ wire (especially common when a heat-damaged connector body collapses); or the sensor element itself has failed shorted. Backprobe the signal at the ECU key-on engine-off and check for the ~0.45V bias — anything over 1.0V is impossible from a zirconia element and points to a harness short. If bias is normal, scan fuel trims — if B1 is running negative trim (subtracting fuel), the rich condition is real and the cat is being damaged; track that root cause before replacing the sensor.

Vehicle-specific patterns: 2002-2007 Honda Accord / Pilot J30/J35 V6 throws P0138 after a leaking valve-cover gasket lets oil drip onto the downstream sensor over time — fix the VCG first, the sensor often recovers after a few highway pulls. 2005-2010 Jeep Grand Cherokee 4.7L sets P0138 when the rear heat shield collapses and traps exhaust heat against the sensor pigtail, melting insulation and shorting signal to heater. 2008-2013 Toyota Highlander / RAV4 2GR-FE throws P0138 from a known-failed batch of Denso downstream sensors (TSB acknowledged). 2007-2014 Ford Edge / Flex 3.5L Cyclone sets P0138 when the bank 1 (rear) cat starts to crumble internally and dumps unburnt HC at the downstream sensor. Cost band: $150-$350 for sensor; $250-$600 if VCG or upstream rich condition has to be fixed first; $1,000+ if the cat is failing.