Nissan Repair: Nissan overdrive, lockup torque converter, electric overdrive

Question
Hi, I recently purchased a 1997 Nissan Altima. Everything has been going fine but yesterday the "Overdrive Off" light came on in my dash and of course it won't shift into overdrive. I pushed the lever in the gear shift a few times to see if I accidentally disengaged it, but that wasn't it. The transmission is not slipping at all the car runs fine but of course it is revving really high especially at speeds over 55mph and it is sucking gas like there is no tomorrow.
Is there anything I can do  before having to have it checked out by a shop, I really don't have the money for it and have some friends who are handy with cars but  not very savvy at diagnosing this issue.
So PLEASE give me a list of things i can do to maybe fix this problem.
Thanks so much

Answer
Marion,

The car has a diagnostic feature that can be read.  You would have to go to a shop to get the code read.  But, the way the electric overdrive works is that the car has to be at operating temperature and you are at speed without being loaded too much.  That is, there is a temperature sensor and a vacuum sensor that let the computer know it is alright to shift into overdrive.  Also, the lockup torque converter applies shortly after the overdrive kicks in.  So, it is probably the solenoid that applies the overdrive, that would be in the transmission on the valve body.  Or, it is the temperature sensor or the vacuum sensor.  Your local transmission shop should do the diagnostic for around $100 bucks