Carbon Monoxide & Flammable Gas Sensor MQ-9 detects the concentrations of CO in the air and ouputs its reading as an analog voltage. The sensor can measure concentrations of 10 to 10,000 ppm for CO and 100 to 10000ppm for flammable gas. The sensor can operate at temperatures from -10 to 50°C and consumes less than 150 mA at 5 V.
Connecting five volts across the heating (H) pins keeps the sensor hot enough to function correctly. Connecting five volts at either the A or B pins causes the sensor to emit an analog voltage on the other pins. A resistive load between the output pins and ground sets the sensitivity of the detector. The resistive load should be calibrated for your particular application using the equations in the datasheet, but a good starting value for the resistor is 20 kΩ.
Pololu Web Site: