I'm having the same problem with my lancer I just bought it on friday according to my connection the recall was done on it. It is throwing the p0421 code as well I don't know what I should replace I've been told replace the cat or say to replace the o2 sensor. Has anyone figured out what to do exactly?

 

Does this failed boot cause a misfire CEL code? I've got that now with my #2 cyl.

P0421 was the cat in my car's case and for a few other people I know. I replaced the O2 sensors hoping for the best, but figured it would be the cat and it was. I was over the mileage for recall, so bought direct fit replacement and did it myself.
If you have to come out of pocket, it's much cheaper to do them yourself, they are easy bolt in for our cars. No welding. I replaced both sensors and both cats for around $500, IIRC.