Brackman,
I think you mean, the thermal fuse mounted on the heater assembly ?

That fuse opens when the dryer temperature exceeds the hi limit thermostat rating.

If you're sure the exhaust vent system is clean, all the way out to the vent hood, your next check will be the blower wheel and housing, especially the lint filter housing and area.

*** A rule of thumb, when you repalce a thermostat, you also need to replace the corresponding thermostat. (in your case the hi limit thermostat and the thermal fuse).***

What controls the heating element, the power to the heater is supplied through both ends of the 240 VAC system.
L2 (120 VAC) is supplied to the heater, through the drive motor switch on the heater circuit.
L1 (120 VAC) is supplied to the heater, Through the timer heat circuit contacts, through the thermostats and thermal fuses.
With 240 VAC the element gets hot, turns red, and produces the heat , until the cycling / regulating thermostat reaches temperature and opens, turning off the L1 supply at the heater element, until the temperature drops, the thermostat closes and the heat cycle begins again.

I would reccomend, you do the checks I mentioned at the start of this post, and replace the hi limit switch, and the thermal fuse kit,

Part number: AP3094224