2021 Toyota Camry: P0420 catalyst efficiency below threshold

P0420 means the downstream oxygen sensor is seeing an exhaust signal too similar to the upstream sensor, so the computer concludes the catalyst is no longer storing and releasing oxygen effectively. On most cars that leads to a bolt-in converter replacement. On the A25A-FKS Camry, the primary catalyst is integrated into the exhaust manifold assembly, so the repair means replacing the manifold and catalyst as one unit rather than cutting in a universal converter.

That changes the economics substantially. Dealer pricing for the manifold-catalyst assembly on the four-cylinder typically runs $1,400 to $2,400 installed. The V6 has two catalysts and roughly doubles that. In California and the states following its emissions rules, only a CARB-compliant replacement will pass inspection, which rules out most cheap aftermarket parts.

• Have the shop graph both oxygen sensor waveforms at steady cruise. A downstream sensor that mirrors the upstream one confirms the catalyst; a lazy, slow-switching downstream sensor is the sensor itself.
• Check for P0300 or a cylinder-specific misfire stored alongside it — a misfire that was ignored is the most common reason a Camry catalyst dies early.
• Catalytic converter theft is a separate and much more obvious failure; if the car suddenly became extremely loud, look underneath before reading codes.
• See catalytic converter replacement cost for how the range breaks down.