Thermistors work by electrical resistance - a low current (approximately 5 volts DC in most cases) is sent up one wire to the thermistor where the current passes through the thermistor and then back down a second wire to the PCB. When the current passes through the thermistor there is resistance and not all the current will pass back down the wire to the PCB. The PCB measures the current it is sending to the thermistor and the current which is returned. Depending on the amount of returning current the PCB decides whether to reduce or increase the gas output to the burner thus modulating the heat input.
The thermistors most common to combination boilers are negative temperature co-efficient (NTC) which means that the hotter they get the less resistance is created in the thermistor and the more current returns to the PCB. Electrical resistance is measured in ohms and thermistors for individual appliances are given an ohm rating usually at room temperature (25 degrees centigrade). When thermistors fail the boiler either does not work or overheats. On older models of combination boilers there will be no fault indicator lights or codes so individual thermistor readings should be taken in order to establish whether faulty or not. On more modern boilers a fault code may well establish the likely culprit!