Absolute pressure sensor outputs a voltage signal corresponding to the pressure in the receiver inlet pipe. ECU checks whether the voltage is within the required limits. If the sensor 4 for continuously outputs the signal voltage is not lower than 4.5 (which corresponds to the pressure in the receiver is not below 114 kPa) or more than 1.95 (which corresponds to the pressure in the receiver is not higher than 50 kPa), the ECU memorizes DTC is R0106 and includes a warning light engine management system.