diplomat adp8224

customer called me out to this machine it was reported not starting when i got there it started straight away ive checked the machine out and all seems ok and hes been running it for a couple of days now with no problem but at the right hand side of the disply screen there is what looks like a heat sign (3 upright wavy lines) with a cross through it there is nothing in the manual any ideas what it is? 😕 😕

thanks paul