The driers I have worked on have a mushroom shaped humidity sensor wired inline wit the element which controls the heat element on auto cycles.
If in a manual setting the drier is fine, then the element itself and timer are likely OK. If the humidity sensor fails the drier either runs on and on with no heat or shuts down early leaving a damp load.
Also try running the drier with the vent pipe disconnected from the wall in case there is a clog causing humidity to back up into the drier , however this is not likely if it works Ok on timed cycles.:thumbsup:
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