P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

The switching frequencies of HO2S-11 and HO2S-12 circuits are monitored by the Powertrain Control Module (PCM). The test fails when the HO2S-12 switching frequency increases to its specified limit value, or the switching frequency ratio of HO2S-11 and HO2S-12 approach a specified limit value.

Possible Causes:
- Pre-catalyst (California only)
- Warm-up three way catalyst
- Exhaust pipe
- Intake air leaks
- Injectors
- Injector leaks
- Fuel pressure too high
- Cylinder misfiring
- Spark plug
- Improper ignition timing
- Oil contamination
- Damaged HO2S
- Downstream HO2S wires improperly connected

Diagnostic Aids:
Check for misfire monitor DTCs.