P0420

Could be the converter is dead, or the o2 sensors, with 186,000 it's way over the 100,000 miles they are designed for. If the downstream 02 is switching too much catalyst efficiency is show to the PCM as being off. Could also be the upstream sensor(before) not switching, but that usually set a code for that! If you have a scan tool the shows live sensor data, check the 02 sensors, the rear, or downstream should remain almost flatline, while the upstream, or front switches back and forth. Bank 1 is on the passenge side.