- Three way catalyst converter Bank 1
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module (ECM)

The most common cause it the three way catalyst converter Bank 1. Changing just the O2 sensor usually don't repair the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

- Three way catalytic converter Bank 1 does not operate properly.
- Three way catalyst converter Bank 1 does not have enough oxygen storage capacity.

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

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).

A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.

When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.

P0420 HONDA
P0420 NISSAN
P0420 SUBARU
P0420 TOYOTA
P0420 LEXUS
P0420 INFINITI
P0420 ACURA
P0420 BMW
P0420 CHRYSLER
P0420 DODGE
P0420 JEEP
P0420 FORD
P0420 HYUNDAI
P0420 ISUZU
P0420 KIA
P0420 MAZDA
P0420 MITSUBISHI
P0420 VOLKSWAGEN
P0420 AUDI