This paper presents the design and development of a Carbon Monoxide (CO) gas detector for vehicles. A gas sensor MQ7 was used to detect if there is any CO gas leakage in a vehicle. To develop a CO Smart Detector that operates systematically, Arduino Uno was used as a microcontroller to control the whole system. The gas detector would automatically alert the user inside the vehicles by triggering a warning using LED, buzzer and LCD display as indicators. An exhaust fan was also used to extract the polluted air from the vehicle. The colour, green, yellow or red would appear on the LED automatically when there is a certain level of CO concentration in the vehicle. Green LED is for safe condition and yellow LED and buzzer with beep sound will be turned on automatically in an alert condition. Finally, red LED with its continuous sound would come on when the MQ7 sensor detects a dangerous level of CO concentration in the vehicle. In addition to the buzzer with its continuous sound to alert users, a fan to extract the air from inside the vehicle could be turned on automatically or manually using a switch if the need arises.