mitsubishi 2001, mirage, 1.8 codes po421

Question

Smog test revail po421. Said to replace front catalitic converter & was done. after 60 miles run , the light again appear on panel. the retest was not done because of light on. what should i do?

Answer 1

P0421 Warm up catalyts efficiency below threshold

For this code condition you may have to replace the front 02 sensor , and if  tune up is due that should also be done.