The upper sensors are used to monitor if the mixture is rich or lean. The lower sensors monitor the pre-cat efficiency. If the computer doesn't see a different reading between the upper and lower sensors, (like if the pre-cat is gutted) it will flag a CEL code. Try cleaning the upper sensors, if that doesn't work you may have to replace them. Might also want to make sure you don't have any intake leakes inboard of the MAF that are really causing a lean condition.
