The temperature probe or the thermostat probe? The thermostat probe should go between the UTH and the underside of the enclosure. If it's on the floor of the enclosure then your snake can move it, dump water on it, pee on it, etc. which will give a falsely low reading to the UTH and cause it to overheat.
You will need an infrared point thermometer or heat gun to measure the surface temperature of the glass under the substrate over the UTH.