Go to another shop. First of all, they should've told you that the code is P0421 - warmup catalyst efficiency below threshold. P0424 isn't a valid code on the Protege5s. That said, it seems as if this is a fairly common problem on higher mileage Protege5s, and 95 times out of 100, the problem is that the catalytic converter near the exhaust manifold has either worn out, or been "killed" by a misfire resulting from neglected spark plugs or bad ignition coils.
It's true that the cat is covered by federal law for 8 years or 80k miles, whichever occurs first. What has me concerned is that fact that your mechanic told you that the initial problem was an intake sensor. This tells me that he either doesn't know what he's doing, or there actually is a problem with one of your intake sensors that is somehow messing up your car, and perhaps contributed to your catalytic converter failure.
Whoever you take the car to, make sure that they figure out if something else caused your cat to fail, or if there's another CEL code. Also, how much are they charging you to repair it? If they are using an OEM cat from Mazda, I would let them know that Walker exhaust makes an aftermarket cat (Part No. 16220) that is about 1/2 the price. Many shops user Walker exhaust parts anyway, so this may be a way for you to save some money.
