If nothing behind the cat can cause a light to come on, why would not closing the gas cap all the way make it? Just something I was wondering.

 

What do you mean? There's plenty of sensors behind the cat that can trigger the SES light.

Also, the fuel system is pressurized. If you leave the cap off, it fails to pressurize the system and triggers the light. I think something to do with emissions and fumes leaking from the car...

 

Why would they bother with anything behind the cat? After that its just a pipe, a muffler, and its out?

 

What are you referring to? Emissions, or anything in general?

Emissions would be the gas cap because the sealed and pressurized system is designed not to leak out fuel fumes and keep emissions from the car in check.

Anything else would be like rear ABS sensors, TCS sensors if so equipped, etc.

 

I was talking about emissions. But you answered my question fine. Thanks.

 

NP