Gas Analog Smoke LPG CO Sensor MQ2

It has a detection range of 300PPM to 10000PPM. You may use this module with a 5v power supply—the resistance of the module changes once the target gas concentration changes.

This module has 4 pins (Digital Pin, Analog Pin, VCC, GND), but you need three (either D or A) to interface this to your Arduino. It needs a 20s heat-up time before it gets the correct data.