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Replace your dryer automatic cycling thermostat, this is what sends a signal to the timer to advance, if this advances in time dry then the automatic thermostat is bad, there is a link below to the video on how to replace this, you can also order the thermostat there for $22.70. I hope this clears it up, thanks, Bryan
Okay it is the timer then. the timer has contacts inside that are bad, if it heats it should advance, is it heating ?