So after a misfiring situation (1, 3 and 6 bad) I replaced plugs and all 6 coils which fixed it. Drove around probably 40 miles with no problem but now it's throwing a new code - low catalyst efficiency on bank 2. Did the misfiring possibly injure the catalyst? Is there any way to clean things or is this some expensive and useless (ie the govt forces you too but it makes nothing work better, just to be in EPA compliance) repair I can't get around?

The engine seems to run smooth i'm mostly annoyed by not being able to tell if a 'new' code drops if the check engine light is always going to be on. :- P

 

What year is your vehicle and how many miles?

I doubt it's your cat unless you drove on bad misfires across country. Check to see if you have an air leak. If you know how to do a smoke test that will help a lot. Secondly if you have a OBD reader that can read what's going on with you O2 sensors check the make sure bank 2 is at .45V ( if it's fluctuating between .1V to .9v then your cat is gone) if not continue troubleshooting. Then I would run a cleaner in the gas and see if it turns the light off. One of those are you issues. I'm leaning towards a good clean or leak.

 

2003 Accord EX V6 with 178k. I put on probably 300 miles in a misfiring condition, I didn't have a choice - I bought it out of town and 10 miles down the road it started, had to get it home, didn't have tools or cash to fix. After I got it home (250mi) I replaced plugs but that didn't fix it, had to limp it to school and back until the ignition coils arrived and I replaced all six.

It ran fine for a good 50 miles without throwing a code after the plugs/coils replacement - it only threw a code in the cold morning startup when rushing to school this morning. Erased it, came right back.

What kind of cleaner would you recommend, I mean fuel injection cleaner or is there something focused on the catalyst? I'm not a mechanic, I can replace parts but don't know things a mechanic should - i'm terrible at diagnosis. Dont know how to do a smoke test and not sure what could have come loose... I didn't fiddle with too much save coils and plugs and the wires to them, unless I might've bumped something.

 

I use Red Line SL1 but you get get some Techron from Walmart of something. You can do a physical check for leaks just by looking. Check for cracks in your air hose coming from the filter box and listen for what sounds like suction. You can YouTube ways to find a leak if you have one. Your cat should be fine and I would look at that as the last possible issue