Surely the safety cut out should be NC and the cooling fan NO? So if both are closed then it's a matter of working out which is duff. As a rule I'd guess against the new one being duff - not impossible of course, but slightly unlikely.
So I do think you've replaced the wrong one.
You said it overran for hours. Does it overrun forever or just hours? Because if it ever stops then it's working - either the thermostat is too sensitive or as TnE said it's staying too hot for too long.
Wouldn't there be some electrikery whereby the thermostat is open and has no power. When power to the over is switched on then a solenoid is closed which allows the thermostat power. When the thermostat warms and then cools sufficiently I would expect it to switch off the fan and push the solenoid the other way disengaging power from itself.
Surely it can be simply left live?
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