DIAGNOSIS The main board is detecting a fault in the heating element circuit. Usually a bad connection at the heating element terminals or a failed element itself.
FIX Unplug the appliance. Let it cool. Remove the pan and basket. Look at the circular heating element in the top of the cooking chamber. Check the two spade connectors pushed onto the element's metal posts. Pull them off, clean the posts with a bit of sandpaper, and push the connectors back on firmly. Plug it in and test.
IF THAT DIDN'T WORK The heating element's internal coil is likely broken. You need a new heating element assembly (part number 421921850401). Requires disassembly of the unit's top housing to replace.
HEADS UP Do not attempt to bypass this error. The safety system is preventing operation because it cannot confirm the heater is working correctly. Operating it with a faulty heater connection can damage the main board.