P0421 Pre-catalyst no longer cleaning properly

By: Diagno editorial team · Updated: 2026-10-15

P0421 means: Pre-catalyst no longer cleaning properly. The catalytic converter cleans the exhaust gases. OK to drive, but have it checked soon. The repair usually costs about €100–1500 in Estonia. The most common cause: catalytic converter is old or worn out. Other possible causes: rear oxygen sensor is faulty, exhaust leak.

What it means

The catalytic converter cleans the exhaust gases. According to the rear oxygen sensor it no longer cleans well enough. The smaller converter sits close to the engine so it warms up quickly. It no longer cleans well enough.

What could be wrong

1. 1Catalytic converter is old or worn out
2. 2Rear oxygen sensor is faulty
3. 3Exhaust leak
4. 4Misfires or oil burning have damaged the converter

What you can check yourself

Before replacing the catalytic converter, have the oxygen sensors and exhaust leaks checked. Quite often the sensor is to blame. Best left to a workshop. It needs a diagnostic tool and experience.

How we fix it

1. We compare the readings of both oxygen sensors
2. We check for exhaust leaks
3. We replace the converter only once everything else checks out

Standard SAE description: Warm Up Catalyst Efficiency Below Threshold (Bank 1)