I recently replaced the Bank 1 upper O2. Then after about 3 weeks the CEL came on again and gave me the codes 171 and 174. What would cause both banks to run lean. I checked my fuel pressure and it is right at 40-44 idleing. I have cleaned my MAF, how can I test the MAF?

1998 Contour SE V6 MTX-75 Mostly Stock 1986 Ranger V6 4X4, the only mod: weight reduction by rust.