| 摘要 |
A gas sensor is provided to detect the concentration of a gas component included in external air which may be led into the compartment of a motor vehicle. When the level of an output signal from the sensor exceeds a first predetermined value, the air-conditioning system is switched to internal air recirculation mode so that unclean external air is not led into the vehicle compartment. When the level of the gas sensor output is continuously kept below a second predetermined value, which is smaller than the first predetermined value, for a predetermined period of time, the system is switched to external air introduction mode. Instead of directly comparing the level of the gas sensor output with the first and second predetermined levels, a relative value of the gas sensor output may be compared therewith. In one embodiment, it is detected whether the motor vehicle travels in a city area or suburb from the varying state of the gas sensor output or its relative value so that the first predetermined value may be set to either higher or lower value.
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