Hello, About two weeks ago my step-son replaced my opti--All was good for about a week--Im going to work and all of a sudden the dreaded "Service engine soon" light came on, went to AutoZone, code was P0420--"The PCM has determined that the catalyst system efficiency for bank 1 is below threshold for the current engine operating conditions.(Bank 1 identifies the location of cylinder #1,while bank 2 identifies the cylinders on the opposite bank.") Im skipping alot of the explanation on causes cuz i figure you guys are looking at the code explantions. What i want to know is--when an Opti is replaced does it cause this problem sometime? My car was running rich before and i hoped the new Opti would help, but it still burns rich out the exhaust(almost think its worst now) and i have an exhaust smell coming through my A/C vent when air is on at stop lights. Is it out of time?? I have the Mac headers with the dual Cat "Y" pipe bought on a GP off this site a few years ago. Thanks for any help--I did do the serch feature--Dwayne

 

Could be a bad O2 sensor. I would also make sure there are no exhaust leaks.

 

Yeah i have an exhaust smell at stops, if a/c is on--I quess this code means my problems are strictly on the driver side--i quess it just happened to happen after opti replacement. Runs a little rough too at stops, even with my baby cam. Would a bad o2 cause this problem even if its after the cat?

 

The rear O2 will not usually cause any drivability problems.