MQ-7 (Carbon Monoxide) Gas Sensor Module
Simple plug and play feature allows you to start recording data by plugging in three cables into any Arduino microcontroller. This unit has four pinouts and an adjustable potentiometer on the backside for adjusting sensitivity. The unit requires 5v DC to operate and can deliver information via its analog and digital pins. Please wait three minutes after powering the device for accurate readings, it may get hot to touch over a long duration of operation.
Resources:
http://playground.arduino.cc/Main/MQGasSensors
Open Arduino IDE > Examples > Basic > AnalogReadSerial