Maybe what he was saying was that there are two oxygen sensors, one before the cat (just after the exhaust manifold), and after the cat. That’s a common configuration. Before continuing, try to get some clarification from the guy on this point. Three might be two cats, but it wouldn’t make sense for a cat to be on the intake manifold. Cats are for cleaning the exhaust gasses, not for cleaning the intake air.
I’m not sure what the DEQ test is but I’m guessing it is an emission test. Deleware Emissions Qualification or something? Anyway, what’s the association with P0421? Do you mean your “check engine” light is on? In most areas if it is on, the car will automatically fail emissions. Try Googling “Mitsubishi Lancer P0421” and see if you find anything. Often if there is a common problem with a particular make/model, other folks in forums around the internet have already discussed it, and Google will find these discussions for you. They may have already found the solution.
There’s other engine problems that could cause this, but based on what you’ve said however, and given the 230K miles you’ve clocked, my first guess would be that you need new O2 sensors and a new cat. New properly calibrated O2 sensors will aid your engine computer to set the proper combustion mixture. A proper combustion mixture goes a long way to cleaning the exhaust gasses even before they get to the cat.
The cat(s) simply cleans up any minor (if the O2 sensors are doing their job) exhaust gas problem remaining after properly calibrated combustion. I think you probably need a new one. They’re supposed to work at least 100K I think, by Federal mandate, but 230K is stretching it for one cat. If you do decide to get a new cat, make certain it is a type that will satisfy your state’s requirements. There’s cats and then there’s cats in other words. The cheaper ones don’t work as well and don’t last as long generally. Some states – like Calif in particular – are very obsessive what kind of cat is permissible, and if you use the wrong kind, you’ll fail the emissions test irrespective of what is coming out of your car’s tailpipe. So double-check that before ok’ing any new cat install work. Best of luck.