Sensor dry says 99 minutes for some settings and counts down but clothes are dry and timer still says 20 minutes

? bad heating element, check out ur thermostats also

Try "timed" cycles. If it shuts off on timed it's not the timer. It's more likely it's not cycling on the thermostat due to an airflow problem like the vent. To rule that out (or in) run ONE load through it with the vent disconnected. If it shuts off you'll know where to look

is it a sensor dryer you have to put wet clothes in a sensor dryer before it works

replace the sensa board and make sure the wires on the sensors are ok

there isn't a timer on this machine,you have a control board,if the time isn't counting down correctly sounds like you have a bad control board that's inside the console

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It could possibly be electrical, check for 240V at the supply side. The only other possibility could be plugged dryer venting, although it would be unusual for it to cause only intermittent problems. The reason the timer would stop turning (mechanical timer) or counting down (digital display), is because on any Auto-dry, (ones that use the moisture sensor) the dryness of the clothing dictates when the timer should count down or advance. Check for good airflow from the outside exhaust point, if that's ok, I would be possibly looking at the outlet for possible loose contact, look at the dryer cord prongs and see if there are dark marks on them, as this may point to a sloppy outlet causing arcing.

just a quick suggestion
check your exhaust outside for proper airflow

We have been having this same problem. I had tried adjusting everything that I could think of when putting a load of laundry in and got the exact same results as you. I would select the sensor dry and it would count down just like normal until it got to 6 minutes and then it would just run forever at 6 minutes.
My husband and I searched online all this afternoon and came across what seemed way to simple to work. We removed the lint filter, unscrewed the filter housing right inside the door of the dryer, and got to the sensors. We took a damp cloth and wiped them off very well to remove any and all buildup. We wiped everything down very well to remove any built up lint and/or dirt and put everything back in place as was before. We then unplugged the dryer for about a minute and then plugged back in. We put a load of already dry clothes into the dryer and started as usual with the sensor dry. It started out at the usual 41 minutes then skipped down almost immediately to 6 minutes and then a few minutes later I heard the most beautiful sound I have heard in a long time. The Beeper signalling that the timer had counted down to the end and was finished. It had worked. Thankfully. Hope this helps you also.

It sounds like you may need a new timer. Has water or anything gotten into the timer. This could be causing a short or faulty reading in it.