I have this controller for my diesel heater but the heat on button is hard to operate. Can I buy a replacement button? I am able to operate it using the back of another button when I've the controller opened.

your 801 Eberspacher controller is a very popular temp control for airtronic heaters, unfortunately though Eberspacher do not supply repair parts for them. Occasionally in our heater repair workshops we may find ourselves with faulty display units where some parts are salvageable for your needs. Perhaps if you could email sales@butlertechnik.com your requirements - put in the subject line HEATER REPAIR WORKSHOP we could then check for you Regards David