Question 1

What do P0420 and P0430 measure?

Accepted Answer

P0420 and P0430 measure catalyst efficiency below threshold on bank 1 and bank 2. The engine computer compares oxygen-sensor behavior before and after the converter; a code means the monitored catalyst path is not storing oxygen as expected, not that one specific part has already been proven bad.

Question 2

Can an O2 sensor or exhaust leak mimic a failed catalyst?

Accepted Answer

Yes. A lazy downstream O2 sensor can mirror the upstream sensor and make a healthy converter look ineffective, while an exhaust leak can pull fresh oxygen into the sample and distort the comparison. Confirm the sensor signals and exhaust path before condemning the converter.

Question 3

Why should I check the leak and sensor path before replacing the converter?

Accepted Answer

A converter replacement will not fix a leaking exhaust joint, a biased O2 sensor, a wiring fault, or an engine condition that is damaging the catalyst. Confirm the leak and sensor path first so the repair addresses the cause instead of replacing an expensive converter unnecessarily.