There several possible reasons which might cause the thermostat not action:
The temperature rise speed is too fast. For example, the temperature rises to the prescribed value in very short time. Then, the bimetal inside of thermostat has no enough time to reaction. The temperature of bimetal is still much lower than the rated value or outside temperature. So, the thermostat would not action. The first trip off temperature would much higher than the rated value. It is so called overshot. This problem can be solved by controlling the temperature rising speed, or making thermostat action 3 cycles. Then, the thermostat would action properly.
There might be misunderstanding for rated temperature. It means a tolerance range, normally 10℃, instead of the specific value. For example: a thermostat is rated at 100℃, the common action temperature range is 95~105℃. It is not just 100℃ only. So, the action temperature for some units can be 104 or 105℃. These units are qualified products.
The thermostat is malfunctioned. The temperature might be out of tolerance, or the two contacts of thermostat were burned and connected together forever. It would be a serious defection and might cause fire or other safety problems.
