losing mpg

I hope someone can help me, I bought a wrx about 5 months or so ago with a 3in tbe and about 2 months ago it but out a cel for p0139 - rear 02 sensor slow response and I bought a spacer which is what some one on clubwrx told me would solve the problem and it did it cleared after I reset the code it didn't come back on for like 2 more months, then it did again but this time I checked it and it said p0139 so I waited a couple days and it went away all on its own, but it came back but with p0442 - eavporation emission control system malfunction, I know I shouldn't listen to the guy at autozone but he told me it is cause I have a bov on my car, but sense that p0442 code ive been only getting like 10 to 17 mph. Or like 120 to 170 miles on a full tank of gas. What would cause this and how can I fix it? Please help and go easy on me I'm a noob with subaru

 

hurray for reposts!

 

ill give it a bump lol