I have a 96 accord ex that I believe may have an exhaust leak which is causing my intermittent P0420 code.

The cat is newer, and both O2 sensors have been replaced. The engine has piston slap and seems to generally be pretty mechanically noisy, so it is difficult to listen for a leak.

I have heard of other options like blowing air into the exhaust and even the use of colored gas and soapy water is another possilbity. Are there any good options that a DIY mechanic can do to pinpoint these problems?

Appreciate any and all suggestions.

 

If there are other engine issues (related to the noisy engine), these can contribute to a premature failure of the cat. You may want to keep that in mind. Also, the OBD II takes a while to analyze the cat's performance, so while the code/light may take a while to come on, that doesn't mean there is no problem in the mean time.

I've used a mechanic's stethescope to listen for exhaust leaks. I'd do it on a cold engine because heating of the pipes could close a small leak, plug the tailpipe, concentrate on the gasket areas as these are most prone to leak. If you can't get to a difficult area, you may want to attach a piece of tubing to the end of the stethescope.

Also, I know you said the O2 sensors have been replaced, but you may want to make sure they are functioning properly using a scan tool. I've never done that, but it doesn't seem too difficult.

 

I've held a rag over the tailpipe to help find exhaust leaks. Not so hard that you plug it up completely, but the excess back-pressure will make the leaks easier to hear & find. Have a friend hold the rag while you crawl around & listen.