What could cause a gas smell inside the car only when the fan/AC/heat is on?

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Noticed a gas fume smell inside my car after a long car trip. I only smell it when either the fan/AC/heat is on. Other than the smell, the car is running fine. None of the dashboard lights are on and my fuel level is not dropping rapidly. Took it to the mechanic and they are stumped. The mechanic thought he identified the smell when the hood was open - they changed the o-rings in the fuel injectors and also replaced two of the fuel injectors. I picked up my car the other day and the smell is still there when the fan/AC/heat is on. I've read up on this online, and most people who smell gas fumes in their car also have check engine/O2 sensor lights that are going off. But none of my lights are going off. Anyone have any ideas? If you do, please share, it'd be greatly appreciated. My car only has 100,000 miles and I would really like to fix the problem so I can keep driving my car!