Check engine light came on

Check engine light came on. Turned on to highway increased speed to 60 mph and noticed the RPM was at aprox. 4000. I then slowed down and noticed the transmission was not changing to overdrive. I stopped and checked the shifter, checked the overdrive off button, then tried again with no shift to OD. I stopped again and exercized the snow and power switch. I then started again and the transmission shifted into OD so I continued home. At home I disconnected the battery and the check engine light reset. For the next two days the transmission worked well. Today the overdrive failed again. I put the transmission selector in snow position and it would not transfer to OD so I continued home.
What could be the problem with failure to transfer to overdrive?
Before I change the speed sensor on the transmission I would like some feedback.