Possible causes

– Catalytic converter deterioration or malfunction
– Exhaust system leaks
– Loose front or rear HO2Ss
– Faulty front heater O2 sensor
– O2 sensors harness is open or shorted
– O2 sensors poor electrical connection

Tech notes

As the code description implies the P0421 code means that the vehicle’s control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0421 code is combined with other codes, try fixing the other codes first.

When is the code detected?

The P0421 code is set when the Powertrain Control Module (PCM) detects rear HO2S value difference becomes closer to value of front HO2S.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0421 KIA Description

The catalyst