Your IP address and browser information will be recorded as proof of this acknowledgement. This data is retained for 12 months.
P0420 is one of the most common fault codes. It indicates the catalytic converter on bank 1 is not working efficiently enough according to the downstream oxygen sensor. This fault code is stored when the engine control unit detects a specific fault condition. Common causes include sensor failures, wiring faults, and component wear. Always read additional fault codes before replacing parts — multiple codes together provide a clearer diagnosis. Clear the code after repair and test drive to confirm the fault does not return.
Based on 0 driver confirmations
No commonly reported misdiagnoses for this fault yet. If you have replaced a part that did not fix this problem, submit a wrong diagnosis report — it helps other drivers avoid the same costly mistake.
P0420 on a car with 110k miles and full history. O2 sensors tested fine. Cat was rattling slightly too — internal substrate breaking up. Replaced with a quality aftermarket cat, not a cheap one. Code gone, MOT emissions passed.
Parts replaced: Catalytic converter (quality aftermarket)
P0420 came up. Everyone said it'd be the catalytic converter and quoted £400-800. Before doing that a mechanic tested the O2 sensor waveform — the upstream sensor was lazy, not switching fast enough, giving a false P0420. New sensor was £55 fitted. Code hasn't returned in 6 months. Don't just replace the cat on P0420.
Parts replaced: Upstream oxygen sensor
Your real-world fix helps thousands of drivers worldwide. Takes 2 minutes.