How does the autopilot act when it receives an obviously bad temperature value? Assuming the plane is on autopilot: for example, if a plane is flying high (over 20000 feet) and its autopilot receives a positive value of external temperature, which is clearly abnormal or impossible.

What will the autopilot do with such a obviously wrong information?
The autopilot does not receive information about the outside temperature, it has no (direct) relation to what the autopilot controls. 

The autopilot receives aircraft state inputs like airspeed, Mach number, vertical speed, attitude, accelerations etc.