The throttle position sensor (TPS) is a rheostat. It is supplied w/ a voltage that the computer uses as a reference. As the throttle is opened, the rheostat causes the electrical resistance to change, which in turn causes the voltage to change (remember Ohm's Law: V=IR, where I equals current). So, the computer continuously gets a changing voltage which it uses to determine the position of the throttle. However, the computer knows the lowest and highest voltages should be, so if the returned voltage is out of range it throws a code. I think you need to replace your TPS. You can verify this by back-probing with a digital VOM and checking the voltage at idle and the wide open positions (key on, engine not running) and comparing those readings to the specs. Also (don't need the specs for this) you can check that the resistance changes smoothly as the throttle is opened (key off). In my experience, when the TPS goes out, there will not be a smooth change in resistance as the throttle is opened - it will suddenly change.