1998 Chrysler Cirrus Troubleshooting codes

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Beginning with '96 models, all cars and trucks sold in the U.S. Have additional oxygen sensors after the catalytic converters to monitor them for proper operation. Based on those readings, the Engine Computer determined one of them isn't cleaning up the exhaust gases. Your mechanic will do a few tests first but most likely the catalytic converter will need to be replaced.