Q. In my i20 vehicle the AC automatically cut off after 45 minutes of driving what is the problem?

It might be that the temperature set by you is very low and during winters temperature achieves fast so it automatically cutoff. This feature is present in car. Also AC cutoff when the RPM of the car is below 800.

It must achieved set temperature of vehicle but automatically start once temp. goes down.

If u put ac on auto mode once temp is maintain ur ac cutoff