I rencently had a problem with my super reliable 1985 Nissan Maxima. I drove somewhere with no problem, but when I left, the (automatic) transmission would not shift out of first to second gear no matter where the shifter was set. It slips some when set to D (drive), but not really when set to 1st. It has been that way ever since. The history of the transmission is as follows: I bought the car @ about 80k miles. All has been well. I flushed the transmission with Mobil 1 synthetic ATF at 155k. Now at about 162k things are broken. For this particular problem, the service manual says the following are possible causes in order of probability:
1. Control cable
2. Throttle wire
3. Oil quality
4. Control valve
5. Govenor valve
6. Band brake
7. Oil passage leak
I've checked and adjusted items 1 and 2 with no success. I checked the fluid level carefully for item 3. It is not burned or particularly dirty after 7k miles and synthetic ATF. Items 4-7 require some significant disassembly. I am probably capable of the following items if needed:
A. Have transmission professionally flushed.
B. Replacing the valve body with a remanufactured unit.
C. Inspect and replace governor shaft assembly.
As these items all require some significant time and/or money, is there one I should try first or is it most likely a failure in the clutches/brakes that I won't want to mess with? Is it time to dump the reliable Maxima? Any suggestions or experience is welcomed.

 

Your problem is tipical of a transmission that has gone too long before its oil change. The new oil, if a good one, will clean up much of the varnish and clog the filter, a valve, or sensor. Change the oil again, cleaning or changing the filter. Normally this fixes the problem.
And don't wait so long for the next change.