The AT indicator illuminates when there is an electrical issue with the transmission or the TCM. This can be caused by poor power, dirty transmission fluid, and filter, wiring, or faulty connectors. Start by making sure your battery is not past its expiration date. A poor battery or poor grounds can cause all types of issues. Get the vehicle scanned, pull the codes, and start to address each code individually.
Changing the transmission fluid and filter can remove some codes that may be stored in the TCM. Back probe the connector to ensure the signal is good going to the TCM and check the fuses. Also, ensure the connectors and pins are free from debris, dirt, grease/oil (electrical cleaner can assist). Check your transmission speed sensor, solenoids, and your oil pressure switch to the transmission as well.
Relocate the TCM to the firewall. The TCM sits on top of the transmission and the heat from the transmission is usually the culprit for a failing TCM. Try the hard battery reset (guide attached) with both negative and positive terminals constantly touching for 30-40min and the battery removed. The hard battery rest if done properly will clear soft codes leaving hard codes if any.
Depending on the code you're getting the TCM can be at fault. Considering the amount of mileage that is on the vehicle, regular maintenance is a must. We recommend getting a reputable professional to properly diagnose a failing transmission and\or a TCM.