Recently I purchased a few gas sensors from Sparkfun Electronics for use in a couple of projects. Most of the gas sensors have a pin layout which makes them hard-to-impossible to fit into a breadboard. To solve this, Sparkfun sells a Gas Sensor Breakout Board to consolidate the six pins on gas sensors down to four.
I procured a few of those boards with my sensors; after connecting everything together, I was a bit unsure how to wire the sensors to my Arduino. There are technical manuals and a few opinions available on the internet but none of them seemed clear. After some experimentation, I found a wiring scheme which appears to work well and I thought it might be helpful to share it.
Some of the sensors may have slightly different needs but I think this will work for all of the MQ* sensors Sparkfun sells.
As a final note, you should let the sensors “burn in” based on their spec sheets. My CO2 sensor has been on for ~12 hours and has shown a noticeable change in sensitivity for the better.