hnwl6136 hoover washer dryer

This is a washer dryer with sensor dry option on the dryer side.

Fault: overheat stat keeps operating.

I have changed ntc.
cleaned the whole system out.
checked wiring.
checked blower motor isn’t running slow.
in fact i watched the dryer on extra dry mode for a whole hour, the heater was being switched on and off as it should.

the customer has again phoned to say the dryer has no heat. 🙁

any thoughts please, has anyone had the small module at back of machine play up????

cheers.