to the manufacturer, DESA International:
To check switch,
set the ohmmeter on the “RX1” scale. With heater cool,
check for continuity across the thermal switch by
attaching the ohmmeter to the
blue wires on switch. The ohmmeter should read "hot" until the ambient
temperature reaches 85F, at which time the meter will read "dead".
If the switch shows "dead" at any other temperature range, then the
part is defective and should be replaced.