My dad just called me up, and said the check engine light on our 02' impala (3400) is on.

He said he saw the gas guage go to half tank, then all the way down below E, and i think he said it is back to normal now.

I assume this has something to do with the fuel pressure or something?

You can take it to AutoZone or O'relly and they can check the code. I found that out after paying $80.00 for a $2.00 PVC valve at the dealer.
You will get a check engine light on for the PVC valve sticking.
Never heard of the fuel gage problem I had a problem with the temp gage doing that and it was the head gaskets leaking (known problem but not admitted by GM).

The light went off. It was mostlikely the gascap was on wrong or something (or had a leak)

I was hoping advance auto parts would do it, cuz the closest auto zone is really far away from me, and the closest oriely is like a 7 hour drive ;)