if anything the lack of back pressure would clean out your cat. your problem sounds like your muffler is flowing so well ( less back pressure ) , your cat is running cooler which messes with O2 sensors and emissions systems. trust me 90% of people that start tunning their honda drive it daily with codes going off. I do everyday but my tuning gives me more horse power and better gas milaege so i live with the codes. im a maintence freak to but my car runs better with my modes

 

I"m bummed that this thread died. I was beginning to think that you were the one who was going to get to the bottom of the problem. All the threads on this subject seem to dead end.

 

Check the FIA (Fuel Injection Air) Control valve located on the thermostat housing. It is only used on 99-00 d16y8

 

Hey sorry about your troubles, I’m more of a sohc expert now. After all my problems and repairs my fix turned out to be a combination of timing belt slipped a tooth on cam and distributer cap. I still have code pop up for o2 sensors cause I’ve gutted my cat, just updating have over 400,000 km and still running strong, power is down a little but considering mileage fuel consumption is great. It’s only my daily in the winter up here in Canada. I’m hoping for 500,000 with regular maintenance. I also found out that when my civic spit out a spark plug climbing a hill going snowboarding, when we disconnected the injector, the engine light blinks continuously. We disconnected it to limp to the nearest town put new plugs and a set of wires that were destroyed and run fine all way back lol