If the timer is continuously advancing even through the off position, then you could have a failed timer or a problem with the motor centrifugal switch. If the dryer stops briefly when it gets to the off position, this would point to the motor centrifugal switch. If it never stops and the timer and motor continuously run through the stop position, then you likely have a stuck contact in the timer.
If the timer does not advance at all, then you could have a failed timer or a wiring failure.
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