Air con problems 2007 Vito cd115

Weird fault with my Vito. Airconditioner will pull the temperature down to 5 degrees (C) then after a short time it shuts down and then reverts to the ambient temperature (34 degrees today. I am thinking the fault is with the temperature sensor in the dash!! Have any members had the same fault and more importantly what is the cure ??????????????????????? Thank you. Frank Baker Geelong Australia.