Intermittent P0421

[goodoleboy]

99 Maxda Protege, 5 speed, one owner, well maintained, 230K miles, and starts easily, runs fine, and consistently gets 30-35 MPG, minor fender bender (radiator, sheet metal, & head light), but the CEL appears intermittently with the P0421 code when I drive it gently. Otherwise. it's fine! The problem has existed for YEARS (mostly a PITA) so I started adding 44k regularly as a preventive about 20K miles ago.

I've seen articles / videos on Seafoam, Marvel, kerosene(?), etc to clean the catalytic converter with a variety of results depending on who you ask. Before I dive into removing / manually cleaning / replacing the CAT, are any of these goo's any good and are there other electronic components which will trigger the CEL / P0421 under these conditions described?

goodoleboy

 

[Lazy2.5]

it could be an exhaust leak

it could be the o2 sensors. try replacing them. would help anyways.

 

[goodoleboy]

I replaced both O2 sensors a couple years ago.

Ideas on how to check for exhaust leak?

Thanks
goodoleboy

 

[misnblu]

I replaced both O2 sensors a couple years ago.

Ideas on how to check for exhaust leak?

Thanks
goodoleboy

Exhaust leak will show itself with black soot around the leaking area.
You can't miss it but you'll have to get under the car to be able to check.

If where you live doesn't give a 'sniff test' of the exhaust in an inspection then I'd gut the cats.

 

[Lazy2.5]

You should be able to hear it. It probably would cause you issues only if it is a significant exhaust leak.

If your protege consumes a lot of oil or has some sort of very rich running condition, it's possible you have got your newer o2 sensors dirty.

So, unless you just so happen to have a bad exhaust leak causing the intermittent code, it's likely a failing cat.

And, I strongly recommend against @misnblu 's advice; with all due respect. We don't need more irresponsible owners sh*tting all over the environment with catless vehicles, especially when your car won't even remotely run correctly if you do so, without getting actual catless headers and a tune.

If it is the cat, you will have to try cleaning it out or replacing it.