Overdrive not working
 

1997. Automatic. Tranny has been shifting fine for 180,000 miles so far. All of a sudden... no overdrive (4th gear)
On the highway doing 60 my RPM'S are now a little under 3000. They used to be a little under 2000.
If I shut the overdrive off nothing happens. The light works.. just nothing happens which means to me.. the car is shifting up to 3rd and the torque converter isn't locking up to get it into 4th.
I did a ton of research on this forum so I will answer the typical question...
There are no codes present. The check engine light is on but its a P0174 which I am hoping is an O2 sensor that I replaced. I am just waiting for it to clear.
From what I read it seems that it might be the Transmission Temperature Sensor?
Transmission fluid is normal looking in the car. It shifts perfectly. It just doesn't go into overdrive.
If it is the transmission temp sensor.. is this something that is easily replaced. Its wintertime here in Connecticut so climbing under the car sucks but I don't mind doing it.
Wouldn't the computer throw a code if the temp sensor was bad? Mine isn't.

While I don't know what's causing it, it seems your slushbox is not shifting to 4th, not just not locking up the TC.

3rd - 60 mph is ~ 3000 rpm
4th - 60 mph is ~ 2500 rpm
4th with TC locked - 60 mph is ~ 2000 rpm

FWIW my 97 always shifts all the way to 4th in the cold, it just doesn't lock the TC until the ATF is hot (above 159F or something like that).

It could be some dirt in the valve body.