Where do I look?

Three months ago I probably made the mistake of buying a replacement secondary air pump on ebay. Now I have the CEL coming on at random but frequent intervals, OBD reports no faults but when cleared the CEL stays off. Bench test of the pump shows it running slow, almost grinding to a halt, so a replacement pump is on the cards. BUT a suggestion from  a Subaru specialist that fault code P0410 might come up if the lambda sensor is failing or has a wiring fault. The question is where on the car are they? Apparently there are front and rear ones, where do I have to get to to inspect them? Previous experience of other makes is that they are carefully hidden so that dealers can charge lots of workshop time just to get to them.