engine malfunction light still on

What is your question?

I have taken my car 3x to the same mechanic in the past 2 months due to the engine malfunction light always being on. My mechanic ran the diagnostics each time and believed the problem to be the thermostat sensor which he replaced on the second visit. The light still will not go off - the car runs fine but I am worried that a hidden problem will become a huge problem soon. What am I missing or what should I ask my mechanic to look for to resolve the issue?