Transmission "AT" on dashboard?

Since my vehicle listed above s hatchback been down for a while, so I turn it on so the battery won't die, then one day I leave it running for a good hour or so with the heater on so the bugs won't get in and then the next day the transmission started to kicks off and not shifting as it should, so I then see the "AT" on the dashboard!

After a few days, turning it on the "AT" on the dashboard was gone and I was like, "sweet! I fixed it lol!" so was able to drive it like normal and then it came back!

Sometimes after running for a long time the "AT" on the dashboard goes away, so I figured that it should be the transmission sensors.

I just don't know where the sensors are located and should I get them at the dealership or just online, according to mechanic Scotty says that those sensors are pretty complex, and you are better getting them at the dealership of your car.

What do you think about this situation?

Thanks in advance!