Abstract: A method of controlling a three state apparatus such as an air compressor comprises a first actuating element e.g. a motor and a second actuating element e.g. a valve, said method including the steps of sensing a variable parameter such as temperature with a sensing means whilst the apparatus is operating in a first operational state, feeding a signal produced by the sensing means to a control device, comparing the signal in the control device with datum information to detect whether the variable parameter is at a value at or above a threshold value, sending a signal to the first actuating element when the variable parameter is below the threshold value whereby the state of operation of the apparatus is changed to an off operational state, sending a signal to the second actuating element when the variable parameter is at a value at or above the threshold value whereby the state of operation of the apparatus is changed to a low load operational state.