P0420: what to test before replacing the catalytic converter
A practical diagnosis order for catalyst-efficiency faults, avoiding unnecessary sensor or converter replacement.
1. Deal with other faults first
Record all codes. Diagnose misfires, fuel-trim faults and oil or coolant consumption first: they can distort catalyst readings or damage a replacement converter.
If the engine light flashes or the engine runs roughly, stop driving and arrange assistance.
2. Check leaks and live data
Ask for an exhaust-leak check and inspection of sensor wiring and connectors. An exhaust leak can affect oxygen readings.
A technician should evaluate upstream and downstream sensor behaviour and catalyst performance using the procedure for that engine. Similar waveforms alone are not a complete diagnosis.
3. Approve a repair with evidence
Ask what test proves the converter or sensor is faulty. Do not automatically replace the rear sensor because it is cheaper.
Confirm emissions approval, vehicle fitment, warranty and the total labour-inclusive quote. Repair the underlying cause before fitting a new converter.