It doesn't stay on, but when it comes on it heats up to between 98 and 105 before it shuts off again. That was why I turned it down. The probe is between the UTH and the glass, under the tank. The temp on the glass with my temp gun would also get to 98-100. That's why I turned it down, hoping that the UTH shuts back off before it get's that warm, but then it doesn't get to 90 on the substrate or the glass that way.