Causes for code 420

My 97 Taurus was missing on cylinder 2 (code 302), lean on bank 1 (171) and had insuffcient EGR flow (401). I cleaned the EGR passage way and replaced the coil because there was no spark on 2 even though there was spark on 6. Doing that solved those problems and the check engine light stayed off for a few days. It came back on and now I have a code 420 for Catalyst System Efficiency Below Threshold (Bank 1). My question is what could have caused that? could it be related to running lean on bank 1? could it be realted to a leak in the exaust? (the car now sounds rougher at idle).

Thanks

Jeff

if the car is no longer running lean on bank 1 then i doubt that would be the problem, my guess is that there is a sensor (soory i don't know too much about the newer taurus') after the cat, and it is just telling you that the cat is no longer cleaning the exhaust like it is supposed to so it's telling you that you need a new cat, just a possibility, but good luck

Your theories are both correct. If it was running lean for a period of time, you could have burnt the converter out. If there is an exhaust leak, there is air coming in that the computer cannot account for. So cure the exhaut leak, clear the codes and see what happens. If same code comes back on fairly soon, have diagnostics ran to check the O2 sensors. If both sensors read the same, have the converter replaced. Good luck.

Thanks that sounds like sound advice, will do.

Jeff