LiquidEliteMax

Hi guys, I hope you can help me with an issue I have been having with my car. So when when i turn on the heater it blows out a gas smell that it is so unbearable that i have to forgo the heat during these cold winter days. I have brought it to my mechanic and he couldn't figure out the problem. My cabin air filter has been changed and my car is fully maintained throughout the years. What do you guys think is causing this? Thanks!!!