P0420 Catalyst 1/1 Efficiency Failure

SYMPTOM
P-0420 1/1 CATALYST CONVERTER EFFICIENCY

WHEN MONITORED
After Engine warm-up to 147 deg. F, 180 seconds of open Throttle operation, at a speed greater than 20 mph, with the Engine at 1200-1700 rpm and Map vacuum between 15.0 and 21.0 inches of Mercury (Hg).

SET CONDITION
As Catalyst efficiency deteriorates, the Switch rate of the Downstream O2 Sensor approaches that of the Upstream O2 Sensor. If at any point during the test the Switch ratio reaches a pre-determined value, a Counter is incremented by one.

POSSIBLE CAUSES
- Exhaust leak
- Engine mechanical problem
- Catalytic converter defective
- Upstream O2 sensor older than downstream O2 sensor
- Catalyst efficiency failure does not reoccur