I had the same problem back when I added the HotShot intake to the PV exhaust. This works in most cases and is harmless....
Unhook the battery and sit on the break to let any and all charge go away. Hook battery back up and give it a try. This worked for me, although not 100%. The mileage went back up close to its original area. It at least reset the ECM since my Check Engine light had come on after the intake install.
Like I said, it's free, harmless, and worth a try.
Gary Noonan
'01 S/C VX / '18 Forester XT