My service engine light was reset by mechanic 2 weeks ago

[assemboy]

My service engine light was reset by mechanic 2 weeks ago, and it hasn't lit up again... My mechanic recently changed the axle seal for me. The o2 sensor was supposidally bad and needed to be replaced, but ever since he reset it, the light don't come on anymore!!! any ideas what might be going on??? does this mean my sensor is still bad or what??

Thanks guys