Thanks for the help. I looked through five pages of messages before I posted. I saw ac problems and overheat problems, but nothing sounded like what I had. After I posted, my step dad was describing some symptoms, like not warming up completely until hitting slow traffic, that led me to replace the thermostat. I also found a vacuum line that was leaking down around the turbo. Replacing both caused no change in the temp gauge reading erratically and the ac compressor not working. I discovered that I can disconnect the coolant temp sensor and the car won't start. After I reconnect it, the car will start and the ac compressor will engage for a few minutes, and the temp gauge will act normal for a moment (not at the top of the red). Also, I'm hearing a popping sound around the heat shield on the exhaust manifold--one pop every few seconds. I figured it was just the heat shield heating up, but I'd swear that I saw the temp gauge react to the pop--sometimes up and sometimes downward. I'll have the codes scanned and probably replace the coolant temp sensor this week. Does this sound at all familiar to anyone?