How do I know if I need a new throttle?

The check engine light went on in my 97 VW Cabrio a few days before the inspection sticker expired so I brought it to the mechanic in whom I have total blind faith. He checked it out and told me the car would not pass inspection because it needed a new throttle for around $800,in fact, he told me, even after the repair, I would have to drive the car for a couple hundred miles to reset the computer. So parts were ordered and would arrive in a few days. The day before the repair was scheduled, the check engine light went off. I took the car through inspection and it passed. It runs fine. So how can I tell if it needs a new throttle? Should I change mechanics?

Well, it sounds like you at least need a second opinion - use the cartalk mechanic finder to locate another VW mechanic that others trust. How many miles? Often this can be caused by deposits on the ‘throttle body’, which can be cleaned off with the proper solvents. So he may be right, but you’ll want to make sure it’s actually needed, and not just a cleaning, or (with the light now off) nothing at all. The light can come on from something as simple as a loose gas cap.

Do you mean throttle body??

I wouldn’t do anything right now. Wait until the light comes back on. There are several sensors on the throttle body that on some cars can NOT be replaced without replacing the whole throttle body.