P0420

P0420 is the most-Googled OBD code in the world and also the one most likely to result in unnecessary catalytic converter replacement. The code says "bank 1's downstream O2 sensor doesn't see enough oxygen change after the cat" — which CAN mean the cat is dead, but more often means something cheaper is wrong: a small exhaust leak before the cat tricks the downstream sensor into reading the wrong oxygen level; a failing downstream O2 sensor itself reads stuck-rich; an oil-burning engine has glazed the cat with deposits that recover after a fresh oil change + a few highway drives. The diagnostic order on a shop floor: scan O2 sensor live data with the engine warm, looking for the downstream B1S2 to be relatively flat (cat working) or to oscillate just as much as the upstream B1S1 (cat failed or near-failed). If the downstream still oscillates, the cat is suspect but the sensor itself could be the cause — swap B1S2 with a known-good unit before condemning the cat.

Vehicle-specific patterns common in shop work: 2007-2015 Toyota Camry/Corolla with the 2AZ-FE 2.4L often throws P0420 because of the ECM's overly-aggressive cat-monitoring threshold — Toyota issued a TSB extending the warranty on 2AZ catalytic converters to 15 years/150,000 miles in many markets; check that first. 2008-2013 Honda Accord/CR-V/Pilot with the J35 V6 often throws P0420 on bank 1 specifically when the rear bank fuel trim drifts long-lean (vacuum leak at the intake manifold runner, NOT the cat). 2001-2007 Ford Escape/Mazda Tribute with the 3.0L V6 frequently throws P0420 from cracked exhaust manifolds before the cat — the manifold leak is the cause, not the cat itself; replacing the cat without fixing the leak guarantees the code returns within 100 miles. Estimated repair: $0 (TSB-eligible) to $2,000+ (full replacement with OEM-grade cat + new downstream sensor).