The ECM monitors the switching frequency ratio of air fuel ratio (A/F) sensor 1 and heated oxygen sensor 2.
A three way catalyst 1 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 A/F sensor 1 and heated oxygen sensor 2 approaches a specified limit value, the three way catalyst 1 malfunction is diagnosed.

DTC No.
| Trouble diagnosis name
| DTC detecting condition
| Possible cause
|
P0420
| Catalyst system efficiency below threshold (bank 1)
| - Three way catalyst (manifold) does not operate properly.
- Three way catalyst (manifold) does not have enough oxygen storage capacity.
| - Three way catalyst (manifold)
- Exhaust tube
- Intake air leaks
- Fuel injector
- Fuel injector leaks
- Spark plug
- Improper ignition timing
|
P0430
| Catalyst system efficiency below threshold (bank 2)
|