2001 Buick Century evac problem

2001 Buick Cent. V6. check engine light on. Had checked-problem with evac purge valve. Replaced one on top of motor(still on). Had checked again, advised purge valve under left rear fender, replaced(still on). Had a smoke test for leaks done, new valve under fender leaking, replaced again (light still on). Had computer reset, computer shows no problem, light off. Four days later light back on again.???