97 lumina 3.1 o2 sensor and egr valve problem-help!

97 lumina 3.1- i had an egr insufficient code (i think 401), so i replaced the egr valve. it was fine, but then came an po141 o2 sensor code. i hooked that up to a diagnostic tool and shut it off and it was fine for about 400 miles,, then BOTH codes came back on. could these be related to something else (bad converter, head gasket, ?????) please help, i don't want to keep replacing expensive sensors to find out they're not the problem.

The egr passages may be blocked, this can set the o2 code under certain conditions.