1995 Chrysler Concorde emissions

I took my car for the emissions test, it failed. The mechanic told me it was because the "check engine" light came on during the test. I told him it has been coming on for a few years now, that I had it checked at another garage and the mechanic could not find the problem and that if there was no change in the running condition of my car not to worry about it.
anyway, the garage I had the emissions test done did a further test and told me I needed an oxygen sensor, which he replaced. On my way home from the garage after paying $362.00 to geet the problem fixed my "check engine" light came on before I even got home. I am being scammed here because I am a women. I immediately called him and told him he said "oh sometimes that will still happen" what sould I do. Did he fix the problem. How do I know
please help a single women in distress
thank you
debbie