Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove leads from heat exchanger overheat stat
Reset overheat stat by pressing down on the reset button

Observation:

Measure for resistance across the terminals of the heat
exchanger overheat stat.
Expected resistance short circuit, thermostat reset.
Is resistance as expected?
THERMOSTAT HAS RESET - RESISTANCE AS EXPECTED - RETEST APPLIANCE
THERMOSTAT HAS NOT RESET - RESISTANCE NOT AS EXPECTED