When the dryer is started the heater starts to heat up then the fan motor kicks in and cools the thermister down. If you listen carefully the fan motor can be heard.If the fan is jammed you should hear a buzzing noise .

The fault f13 means high heat on dry.If the thermister gets too hot , or thinks it`s too hot then f13 will occur.If the thermister fails then it will throw up f13 or the fan is not blowing ,or not blowing enough.Also check the wiring and condenser for blockages . The main cause on this heater box is a faulty thermister ,or wiring to thermister.If all alse fails replace the module( sometimes the module can heat up the dryer heater when it is washing).

This is another type of heater box.Just the same ,if the thermister gets too hot ,or thinks it`s too hot then f13 will occur.on this type of heater box the main causes are dryer filter blocked ,fan jammed or faulty thermister/wiring , if all else fails replace the module ( sometimes the module can heat up the dryer heater when it is washing).