I have a 3.7 v6 2015 mustang and my engine is running rich. I changed all my spark plugs but there’s still a strong smell of gas that comes from the left side of my engine. I have no codes popping up on my tuner and there’s no lights on my dash either.

 

When did the problem start?
What do the fuel trims look like for both banks of the engine?

If you're smelling it in the engine bay and not the exhaust you probably have an injector leaking. Fuel trims would identify which side.