Miata warm up catelist

I have a 99 Mazda Miata, with the check engine light on. I pulled the code, and it says “warm-up catelist below threshold” The parts guy thinks that means the front o2 sensor. Says this stuff is all “smoke and mirrors” if taken to a dealer.I hate to buy that sensor if it isn’t that. Any insight on what this means?

Please bring the code here. We have our own sources for interpretation. Right now, it’s rather cloudy.

That could should be in the format of “P0123” How about posting it?

Yes, it’s a PO421–If I reset it, the light doesn’t come back on for about 100 miles.

your cat is smoked,as long as no TSBs EXIST,and the O2s swing correctly.plus no air leaks.

Its a po421–is it really the cat???Any other possibilities?

It’s either the cat or the O2 sensor. But, since the light takes 100 miles to set again, which is typical for the proper test sequences to happen, I’m leaning more towards a cat problem. If you need to pass an emissions test in your area, you’ll need to get this replaced in order to pass.

Thanks–no emmission test required where I live. The car runs the same, with no decrease in fuel economy. Since I have been ignoring it for some time, I think I will continue to do the same.

The parts guy is part-ly right! It may be smoke and soot. The oxygen sensor(s) may be soot covered from an engine running too rich. A rich running engine will run just fine, for a while…but, emissions…whoooo boy.
Try the cheap(er) stuff first to get the engine to burn cleaner: Change the spark plugs. Look at them when you take them out. Are they sooty, or have other deposits?
Change the air filter if it not recent. Use an MAF (Mass Air Flow) Sensor Cleaner. Use a Throttle Body Cleaner on the iac (idle air control) passages, and the throttle body bore and throttle plate.
Check the operation of the egr valve. Check,or replace, the ects (engine coolant temperature sensor).
Your engine should, then, run leaner which will send less fuel into the cat.
If this doesn’t do it, it may be the oxygen sensor(s), and/or the catalytic converter. A diagnostic scan can show the health of the oxygen sensors; but, it’s not free.