P1151 Lack Of HO2S21 Switch - Sensor Indicates Lean
Sounds like you found the right elbow
Its more common for a vacuum leak to cause this CEL but not impossible for it to be a bad 02 sensor. If you feel that you have expressed all options as far as hunting down a vacuum leak which could be anywhere from the throttle body gasket, elbow gasket, pcv line, intake tube and a few other places then I would suspect the 02 sensor. Just tough to think one went bad at 87k but its not impossible.
Maybe someone else can shed some light on a possible cause while I