I had it read and the code is
PO171
SYSTEM TOO LEAN
BANK 1
WHAT DOES THAT MEAN.

 

I think that usually means the O2 sensor. Clear the code and see if it comes back quickly. I always reset the CEL and then see if it comes back with the same code. Most of the time I do it a couple times before I actually fix something.

 

But why would it goe bad right after I replaced my old Gas filter.

 

Yes that would be the o2 sensor in bank 1. It could be bad, I think they last about 100,000 miles, -or-
it could be doing whats its supposed to and letting you know that somthing is not mixing right.