I received a P0420 Catalyst System Efficiency below Threshold (Bank 1) code reading. It was suggested I replace the cat converter(s). How do I determine which cat is "bad" or clogged (front or rear)? Also, should I be quick to replace the cat, is there an alternative, replace O2 sensor, cat cleaner(s), etc.?
My car has 171000 miles.
My car has an automatic transmission.
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