P0420 does not indicate impending engine failure. Diagnose catalyst efficiency by comparing front and rear oxygen-sensor waveforms. Likely causes are a failing catalytic converter or faulty oxygen sensors; on a 1.5L engine, also inspect the exhaust manifold from underneath for cracks.
The owner of a 1997 Mazda Protege with 156,000 miles asked whether a P0420 code means eventual engine failure or only a tune-up after repair. Experts explained that P0420 means catalyst system efficiency below threshold on bank 1 and that the engine itself should be fine. They advised checking the catalytic converter and both oxygen sensors, and, if the car has a 1.5L engine, inspecting the exhaust manifold for cracks that can set catalyst and oxygen-sensor codes.
