Thermistors work in conjunction with the user potentiometer (boiler setting for temperature), and the Printed Circuit Board (PCB). The boiler will ignite when the resistance reading drops below a certain level.
What happens when a ntc thermistor goes bad?
As a thermistor is failing, it will show incorrect temperatures. Impossible temperature fluctuations will come up. You must change the broken thermistor immediately. By the way, handling damage, excessive heat, thermal mismatch are the most failures.