What does the code P0126 mean? I had Auto Zone diagnose why the check engine on 2006 Mazda Mazda6

Light came on and the paper they gave me said P0126. Check the radiator and the overflow was empty so I filled it half way and the light went off but the next day the light came back on and the paper said either the thermostat or the sensor and I would like to know which so that I dont have to change parts I dont need to

Asked by for the 2006 Mazda Mazda6
code means thermostat is posss stuck open,but if your loosing coolant have car pressure tested to determine leak at that time rep leak and repl thermostst