A control switch comprises a bellow which contracts and expands pressure of the refrigerant, a lock/unlock member which cooperates with the existence of the pressure of the refrigerant, a spring for forcing the lock/unlock member against the pressure of the bellow, and a rotatable shaft operatively disposed through the lock/unlock member. The lock/unlock member has an unlock slot proximate the bellow, a lock slot distal the bellow, and a tapered transition portion between the unlock slot and the lock slot. The shaft is a polygonal, e.g. hexagonal, which fits into the lock slot when the pressure of the refrigerant is under a predetermined value. The user is made aware of the leakage of the refrigerant by the restriction of the rotation of shaft or a break in the power circuit, or both. This prevents the operation of an inefficient in the air conditioner.