What does OBD code P0420 mean?
P0420 means the catalytic converter efficiency is below the required threshold on Bank 1. The downstream oxygen sensor detects that the cat is not cleaning exhaust gases as effectively as it should. P0420 is one of the most common OBD codes on older vehicles. However, it can also be triggered by a faulty oxygen sensor — always get the sensors tested before replacing the catalytic converter, as the cat is much more expensive.
Likely causes
Worn or failed catalytic converter
Most direct cause — cat has reached end of life, common on vehicles over 100,000 miles
Faulty downstream (post-cat) oxygen sensor
A failed sensor can mimic cat failure — always test sensors before replacing the cat
Engine misfires damaging the cat
Unburned fuel from a misfire enters the cat and overheats it — fix misfire codes first
Exhaust leak before the sensor
Unmetered air entering the exhaust affects sensor readings
Repairs you may need
Typical cost range: £40–£900 — Lambda sensor: £100–250. Catalytic converter: £250–900 depending on vehicle. Always diagnose before replacing the cat.
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