Hi,
This isn't my S series but on my 2000 Yukon xl this issue just started today.
A little background, fluid and filter changed this fall, up to level,red in color,no funny smells and no leaks.I will throw in No Check engine lights (yet).
Now about the issue,I parked the truck 48 hours ago working fine, today took it out for a ride and it wouldn't downshift on its own.Even when stomped just kinda bogged till it got up to speed (I do have a CAI)...
When I move the gear selector manually down it goes threw the gears with no issues, then put in drive goes up threw the gears like it should but didn't downshift automaticlly..
Again, no check engine light or anything to give me a direction...
Could a dirty MAF cause an issue like this? Maybe my K&N oiled it?

 

interested in answers, same issue today on my 94 s10 blazer

 

Well it turns out it is my remote starter causing the codes..
Only happens in the cold weather for whatever reason, but only after I use the remote starter....
I also read up on it and a bad ignition switch can cause a similar issue...