1998 Acura CL code P0420

This code most often turns out to be a bad catalitic converter. An air leak in front of the converter can cause this code as well.

There's 2 HO2S one below rear of engine compartment, in exhaust pipe forward from the TWC Converter. The other one is underside of vehicle in rear of the TWC Converter.

It's a good idea to check the operation of the O2 sensors with a scan tool or scanner software. You should see normal switching activity in both sensors shortly after the engine is started, with the O2 sensor voltage switching back and forth between rich (over 0.8 volts) and lean (less than 0.3 volts). A flat line O2 sensor reading or one that shows little switching activity is a bad sign. The downstream O2 sensor should slow down and go flat when the converter lights off (if the catalyst is working). If it keeps on switching like the upstream sensor, it tells you the catalyst is probably bad and the converter needs to be replaced.