Hello, this is an over temperature switch. Most commonly caused by dirty filter, dirty ac coil above furnace, blocked/insufficient ductwork etc.
It usually resets itself when cool, but yours appears to be stuck.
Behind the gas valve is a flat plate. It has 2 wires attached to it. see if you can identify it and let me know
I understand. It is always possible to just have a bad switch.
Turn power off, remove that switch and tap the opposite end where a round bimetal disk is ( you wont see it until it is removed).
Reinstall and see if fault is removed. If not , put the 2 wires directly together and see if fault clears. If so replace switch.