P0420-How to determine if the rear 02 Sensor OR the Cat is at fault?

google catalytic converter temperature and use an inferred thermometer to measure the temp front and rear of the cat , beat on it do you hear a rattling coming from the inside? yes= sure sign of failure . swap o2 sensors front/rear left/right if problem follows o2 the o2 is suspect. p0420 is a generic code you need a more definite code(manufacturers fault code) to do the o2 swap.

The Easy way to do this is if the front and rear o2 sensors mirror each other in terms of amount of fluctuation and the outlet temperature of the Cat is at or lower than the inlet temp the cat is faulty. ( we use an infrared thermometer and on a healthy cat the outlet temp can be as much as 200 degrees higher). Hope this helps!

For your 98 Town Car, P0420 and P0430 are nearly always reflective of internal catalyst inefficiency, resulting in replacement of the affected converter(s).