Bosch Excell Tumble dryer doesn't heat?

The fan on the dryer sometimes doesn't start up. This causes the heating-element to overheat, tripping the thermal cut-out and thus stopping the tumble dryer heating. The problem looks to me like the temperature sensor that tells the fan to start up is going faulty. There are three temperature themostats: 175 degree safety limiter, 75 degree limiter and 120 degree limiter. The first is the thermal cut-out, which has a little red button on top to reset it. I assume the 120 degree thermostat limiter is the one to switch the dryer fan on and the 75 degree thermostat limiter switches it off. Please can you confirm that the function of 120 degree limiter thermostat is to switch the tumble dryer fan on.