A method and apparatus for determining the sufficiency of refrigerant charge in an air conditioning system by the use of only two temperature measurements. The temperature of the liquid refrigerant leaving the condenser coil is sensed and the temperature of the condenser coil itself is sensed and the difference between these two measurements is calculated to provide an indication of the adequacy of refrigerant charge in the system. This process is refined by steps taken to eliminate measurements during transient operations and by filtering signals to eliminate undesirable noise. A permitted threshold of deviation is calculated by using probability theory.