Why does my check engine light keep going on after replacing the oxygen sensors?

Get the codes read and write down the numbers. I imagine the mechanic has done that in between bouts of head scratching.

Multiple things can happen: 1. You or the mechanic replaced the wrong O2 sensors or AFR sensor. 2. you replaced the right O2 sensor, but there are other issues with system like fuel pressure, evaporation system leak etc. 3. The OBD scanner can read codes as well as clear the code array, your mechanic should be able to do that and figure out the active codes that are causing the check engine light. Hope this helps.