Q: Car independently downshifts on startup

When I was slowing down, my car independently started downshifted very hard. I never shifted the transmission. This problem stopped happening for about a week and then started up again, This time the check engine light came on, but was not flashing. Now the problem is constant, both the hard downshifting and the check engine light stays on. I put it in neutral now when slowing down and this avoids more strain on the transmission. But when I shift into Drive, it hard shifts. Also, my car has a sealed trans fluid unit, so it’s probably not the fluid. I will be taking it to be serviced, but before doing so I want to find out what is wrong with it. What is your assessment of these problems?