Darkcrime

My friend has a 99 cobra and when he pulls up to a light and shifts into neutral, his car will rev up to 3000RPMS and stay there. He has a check engine light but when we ran the code, it had nothing to do with the problem. TPS Voltage was good (.99V). Any idea's? Before we start replacing parts, we want to make sure of the issue before hand.

Thanks guys.