I have a whirlpool electric dryer and the automatic sensor mode is not working. In the automatic sensor mode it drys the clothes but will run forever. The timed mode works by drying the clothes and shutting off at the end of the cycle. I have replaced the timer and the sensor mode still does not shut off. I have cleaned the sensor strips inside the dryer with rubbing alcohol and no change. I have run the dryer without the vent by placing a stocking at the output, no change. What do I do next?

In some dryer's the control panel relies on a thermistor rather than a CYCLING thermostat to regulate the drum's air temperature by monitoring the component's resistance changes; resistance goes down as temperature increases and up when temperature decreases. Once the drum's air temperature reaches a certain level required to dry clothes, the control panel shuts off the heater. The panel will turn the heater on again and begin another heating cycle when the thermistor indicates that more heat is needed to keep the air temperature constant inside the drum

Lastly check your moister sensor. ( located inside the dryer door usually) Especially if machine seems to shut down early and clothes are still wet.
Test with a meter at room temperature and it should show continuity.
A failed moisture sensor will affect the dryer run time in the automatic moisture sensing cycle but it will not affect the heating of the dryer or the timed cycle. Which are reflected by the thermostats.

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if it wont shut off this is a timer problem contacts fused closed,are you sure this is a new timer?(some are remanufactured)

You will need to replace the sensor. Here is the part to order.

Bad sensor, will need to be replaced.

One thing you can do is clean the sensor strips located inside the dryer. Get some windex and clean the coating off them. Dryer sheets rub against the strips and form a barrier. The machine THINKS the clothes must be dry because it ain't SENSING dampness from wet cloths rubbing against them sensors.

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Does this dryer have an automatic mode? If so, does it dry appropriately in manual (timed) mode?

If the timed mode is working, then the moisture sensor is bad.

First try cleaning the moisture sensors with rubbing alcohol, these are two silver colored strips located inside the drier just below the lint cleanout

Before you do that try this pull the unit out from the wall remove the venting from the back run a load and let the hot air go intop the house if it drys fine you have a plugged vent going to the outside and it will need to be cleaned out (This is the most common problem for long dry time)

It is the moisture sensor system, it is not sensing the wet clothes. Open the door, inside you should see a part with 2 metal bars that is around 3 inches long, clean these bars with a mult purpose cleaner. Sometimes fabric softner sheets and detergent builds up on this so that the machine cannot sense the moisture. Hope this helps.

check your venting !!! run a load with out the vent pipe on the back see if it drys normal if so your problem is in you vent duct to the outside