Brother-in-law has a 03 2500 quad cab, long bed, 6spd. only mods are a power pup and straight pipe. he called me just a few minutes ago and said that his CEL is coming on. when he checked it with the power pup it said it was the throttle positioning sensor. the wierd part about all this is that the CEL doesnt come on when the A/C is off. he says the engine doesnt idle up at all when the a/c is turned on. is this a normal problem with these trucks? is it as simple as a bad TPS?

thanks for any help...