My friend's Civic failed emissions, barely. We did some work on the car in hopes of passing:

changed spark plugs
sprayed a half can of carb cleaner
patched a hole in the tailpipe (actually, reattached the muffler)
changed air filter
put techron treatment in the tank

My question... the muffler is almost shot. its got a small rust hole and the inside is falling apart.... but will this effect emissions? I hope that patching the tailpipe will bring C02 level back up, and the air filter change might lean out the mixture and lower the HCL? I dunno I need suggestions. retest is on Wednesday. thanks!

 

 

from what I've found yes it does have an effect... my car still doesn't pass smog... but because I fixed the hole in the muffler (well put on a new full exhaust) it got me closer to passing... so yeah it will help but don't think it'll allow your friend to pass