I took out the stock air box and replaced it with my custom intake, and i also put the sensor in the engine compartment because it wouldnt fit into my "rice" pipe. After i got my catalyic convertor installed, ive been getting the SES light when i get to moderate speeds like 35-50. When i slow down and brake, the light goes off. This is very annoying and i just got the car from an auction.

First off, get that sensor in a good location. Your going to lose power if your IAT is dangling in the engine bay reading heatsoaked air temperatures right beside your exhaust manifold. You want it to actually read the temperature of the incoming air. Make a hole in the base of your filter for great results.

I'm going to assume this is on the '91, and if so you can read the codes your self. Procedure is in tech info . Tell us what you find.