I joined this site to thank ya/ll for the info I found. I was going to post this in the general forum, but it seems I can't. Anyway, I needed to find out what a check engine code means for a friends 98 Accord. It's a 2.3/a4. He had acode come on and we scanned it at Autozone. The sales clerk sold him both new O2 sensors. After reading the print out from Autozone, I had him take the sensors back, as it seemed to me that they were working correctly. BTW, the code was PO420, catalyst system efficiancy below threshold. It seemed to me that if the sensors were saying that the exaust system wasn't working, then the sensors themselves were working, and something else must be wrong. After performing a search on this site, I found that this seems to be a somewhat common problem involving faulty catalytic converters. Unfortunatly, his car has 103,000 mi and the cat is no longer under warranty. I'm going to go ahead and order a new cat for him and install it. Does anyone have knowledge on the average cost of a stock type cat for this car? I'm assuming in the $180-250 range. Does this sound about right? BTW, his car is a bone stock commuter vehicle. Anyway, I 'm sorry the post is so long, but just wanted to say thatnks again, and great site!
Kevin D.

 

it would throw the same code if your o2 sensor are bad, i could be the cat. converter but if you ask me, i am putting my bet on the sensors.