I just took my 1993 in to a mechanic for an estimate. He said that the car is not running in open loop and because of a power failure the car needs to go through a relearning process. Could the culprit be a faulty sensor? They want $240 for a few more hours of testing and reprograming.

When your car is running properly the PCM should be in closed loop. If it is running in open loop there probably is a faulty sensor. Try resetting PCM by disconnecting negative battery cable for a few minutes and reconnect. Drive car for a long drive and PCM will relearn all the sensors.