Run the TCU diagnostics procedure to narrow it down - available in the FSM if you download/purchase it.

Here is NISTECH's description from http://forums.nicoclub.com/zerothread/141499

Quote, originally posted by NISTECH »

Ok AT self diagnostics comin at ya. Start by holding your tongue so it hangs out the left side of your mouth, raise your right eyebrow. lift your left leg off the seat. 1)start vehicle and warm it to operating temp. 2) turn the key off and press the OD button to on position make sure shifter is in park. 3)Turn the key on engine not running make sure the od light comes on for 2 seconds. If it does go to #4 if not post back. 4) turn the key off and shift the trans to D. use the lock release if neccesary. Set the OD switch to off. 5) turn the key on and wait at least 2 seconds. then set OD switch to on. 6) Shift to 1 7) set overdrive switch to off. 8) press accelerator to floor then release it completly 9) set overdrive switch to on. This will start the OD light to flash your codes. The light will have one long illumination flash then start flashing short blips. Watch the flashes carfully cause one or more of them will stay on slightly longer then the others. There are 10 flashes total. If say it goes like this, first the start signal long flash then blip..blip..blink.blip..blip..blip..blip..blip..blip..blip. notice how the thrid was a little longer. The would be a 3 which corrospondes to TPS short or open. When you do it there are 10 posibilities plus a no failure which would be all blips of the light. post back what you have and I can reffrence my manual to see what it is. Reffrence is a 1992 240sx service manual.