Nissan Repair: Transmission, nissan altima, temperature switch

Question
Hi I have a 96 Nissan Altima and have problems with my transmission, it does not shift into auto overdrive. I push the button on the side of the shifter and the light comes on but does not activate. I had a previous short that was fixed by a tech and he said all electrical and E.C.M is ok. it only goes 1st,2nd,and 3rd gear and stops at that. The speedometer works and the rpm meter also works as well. The check engine light is also on. Do I check for further shorts or what can I do? I would appreciate the help and thankyou.

Answer
Pete, there are a couple of sensors for overdrive and the lock up converter to activate properly.  First, operating temperature from a temperature switch on the engine.  Second, a vacuum sensor that needs to see that the engine is not under load.  Check both of these and get back with me on what you find.