Main code 66 indicates the ECU is expecting to get its throttle position sensor (TPS) signal across a serial communication interface from a computer-controlled engine. Either the engine computer is not sending the TPS information or the wiring between the engine and transmission computers has failed.
Code 66 00 can occur when the transmission ECU remains powered when the engine ECM is powered down. The transmission sees this as a communication link failure.
If a Code 66 00 is set and the ECU was programmed after 9/26/94, the SERVICE icon on the shift selector will be illuminated.