The engine light came on,problem was fixed, but the engine light is still on.WHY on 2008 Chrysler 300

A diagnostic check determined the thermostat in the car was going bad,and that was fixed but the engine light is still on ,what do I have to do for the light to go off.

Asked by for the 2008 Chrysler 300
If the light's on for the same code , it wasn't diagnosed .WHAT was the SPECIFIC code? Coolant temp sensor out of range/poor performance? Give a # code EX. P0XXX.
I got a diagnostic @ Meineke the code is P0128/181 coolant thermostat stuck open,a new thermostat was put on,but the engine light is still on,why?
Because they tried to fix the code not diagnose the cause of the code. I know it says stuck open , but it's using a sensor and comparative values in a computer. maybe you got a bad thermostat or it's in backwards. Air pocket at sensor. Other reason engine is not warming up properly (which would HAVE to be the case if the thermostat was truly stuck open.
the light did go off a few days later. THANK YOU