1999 Toyota Camry EMISSIONS TEST

I took my mother's car to a smog test/ repair station and it failed. The check engine light had been on for several months. The mechanic ran a diagnostic test and it showed the oxygen sensor was not working properly. The oxygen sensor was replaced 2200 miles ago. He put a new oxygen sensor on and it greatly improved the emissions but still will not pass. He is stumped because another diagnostic test shows everything working properly. The engine sounds and runs perfectly. There is no visible smoke coming out of the tailpipe. He said he has checked all the sensors and functions.