Hoping that this makes it easy for someone else to find this thread because it took me a while to make this link.

My issue was caused by a faulty gear position sensor. I replaced that and the 3rd and 4th pressure sensor switches and it has been running without any issues for a month.

never heard of a gear position sensor. I will look it up

Update:
It has been little over a month and no problems so far. I drive about 50 miles every day to-and-from work.

One of my co-workers bought a TL in the summer, and he did mention having a similar problem, except instead of downshifting, his car went into neutral. What transmission you have? he has 6sp MT

^I'm gonna guess the OP has an Auto transmission. I have never heard of a MT transmission downshifting on its own!


Thanks for the follow up/update MS4246!

When your car downshifted, did the VSA and /!\ light come on as well?

My car will downshift and basically go into a limp mode, power is severely limited and it won't let me shift with sport mode. When I decelerate, it will downshift on its own. I haven't tried starting from a stop while its in this state but I suspect it would not leave 1st gear.

The two times it has done this to me I stop ASAP and turn the car off and back on and it goes back to normal, with the VSA and /!\ light off.

He describe the 3rd gear syncro issues causing the gear popping out.

Last year I thought my automatic transmission was destroyed as it downshifted 1 or 2 gears by itself on the highway and it wouldn't let me upshift in manual mode. Once in the city driving it wouldn't go past 3rd gear from what I remember. Considering how far my commute was to work, I was considering selling the car and possibly finding a job closer to home during this situation. Thank you OP. Read this thread last year, did the pressure switches as well as the gear position sensor and AT fluid 3x3. My car has been running great ever since.

did your tl ever not let you shift to the next gear or shift for you while in manual mode? When im in 3rd gear and hit about 5k rpms it shifts automatically while in manual mode sometimes. Did yours do this too before you replaced the sensor?