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.