ITC-308 Not turning on heating element

I have a 4 year old ITC-308 that has worked in the past. It is properly reading the temperature, but is not turning on the heating element despite there being a large differential in the desired vs actual temperature. The heating element works as intended when plugged into an outlet by itself. I tried unplugging and plugging it back it while holding the set button to enter test mode but that didn’t do anything after I hit the up or down button. Is there anything else I can try? Thank you

Sorry, If the heat indicator lights up while in test mode, but the heat output will not turn on, there may be a problem with the ITC-308.