Wife has a 2002 Elantra with automatic transmission that was purchased new
and has been trouble free since today. She stated that the check engine
light went on and the rpms on the tach increased significantly but the
vehicle would only allow a maximum speed of 20 mph. She then pulled over as
the engine was revving too high and shut the ignition off and called me. I
was able to drive the car home without incident and none of the symptoms the
wife had encountered. The check engine light was still on though.

I scanned for trouble codes and came up with one: engine PO121 Throttle
Position Sensor  implausible signal. No codes for the transmission.

I cleared the code and it did not return after starting and shutting the
engine off several times. What did happen as I placed the transmission into
drive or reverse was the speedometer jumping from 0 - 140 while barely
moving in the driveway. It would stay at 140mph and not return to zero until
I either placed the vehicle into drive or reverse again. The needle just
keeps swinging back and forth erratically.

Any assistance would be appreciated.

Thanks