losing gas mileage

I have a 1999 Chevy Venture. I have been getting 19-25 miles per gallon. In the past few months my mileage has dropped to 13-15 miles per gallon. I picked up a fuel filter, but need to figure out how to relieve the fuel pressure to replace the fuel filter. I had my code read(fix engine light is on) and the code read "lean gas". Doesn't this mean Its not using that much gas. But, my car is actually using more gas. Does this make sense to anyone? :banghead:

Could you tell us what the code number is? It will help us figure out what's wrong.

I believe the code said 171.

You might want to check the following.

Inspect the air intake ducts for being collapsed, damaged areas, looseness, improper installation, or leaking, especially between the mass air flow (MAF) sensor and the throttle body.

This could be causing a lean condition.