2001 Mazda Protege 4 dr, loaded, 2.0, etc. Keep getting the old PO421 CEL code, i.e. Bad o2 sensor (one, two bad sensors or that AND the pre-cat), and was wondering what luck anyone has had with any of the "cat cleaner" products out there. Amazon ratings are mixed: if the cat's shot, nothing is going to save it, I know...but here in California it's a $1700 job on a protege with 212,000 miles. [Just put $1400 in it for new cam seals, belt, pulleys, tensionser, etc and a radiator. Merry Xmas, eh? Previous guy did not do it right.]

Cat and one sensor (can't remember which) was replaced 100,000 ago after one of those famous Protege coil failures, but have had one coil misfire/replacement (both replaced) since then a coupla years ago...taken care of, but afraid it might have clogged/poisoned my cat.

In any case, can any of these after-market products HARM a high-mileage engine?

(I just hate taking the thing in and blowing $100+ clams for a "diagnosis" until I'm on sure ground.)

Thanks to the group.

-Broke in San Diego