On the diagram, how hard is it to get to the Ho2S-21 sensor? I must have missed that. The only sensor I located was way down below the exhaust and was very visible. I went out to my truck and I can't see any upstream sensor (maybe it is hidden?) I notice that on your diagram there is an * stating that Many applications use only one (1) downstream HO2S, which may be why I only see that one? Thanks!