Replace computer or additional diagnosis? Computer program requires dealer only?

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Problem-rough idle, stalling, rough running . Repair#1-accessory belts, shocks replaced; knock sensor replaced. After repairs service engine soon light illuminated. Told wires shorted- need a computer Repair#2- has a code for idle air control-valve will not operate; wires damaged at the connector, shorted. Repair wire harness and installed new idle control unit. System not working properly-computer will not operate valve on demand. Referred to Nissan for computer reprogram or replace. Service #3-Nissan suggested extensive diagnosis (follow the wires) to determine problem. Est 4 hours to diagnose Is it true that only Nissan dealer can program computer?