Transmission troubleshooting

Okay. Go easy on me. Been on vacation.

Day before I leave, my son calls, his 2006 Aveo is "driving slowly"

Get a little more information out of him and it looks like the Aveo dropped into 2nd gear to protect the transmission.

I had it towed to the house, have not touched it. Tonight will be the first time I take a look at it.

Going to check the fluid levels, and fuses (if there are any related to the transmission?). If the fluid levels are good and the fluid doesn't smell burnt or off, I am off to phase 2 of troubleshooting..

While I am out there, are there any other simple or obvious things that could be wrong? Does this transmission have a filter?

What are some other Transmission issues that have been out in the wild for an '06 Aveo?

Thanks!