On my way home today my cel came on and I noticed my tranny would not dowshift automaticlly and I had no power in o/d.I checked the code and it was P120/tps.The light went off after I checked the code.Should I replace the sensor now or wait and see if it comes on again?It drives like it did before.

 

There should be a FSM testing procedure of the TPS. I would try that first.

*Edit* Here it is:

Detach the Throttle Position (TP) sensor connector.
Apply vacuum to the throttle opener.
Using an ohmmeter, measure the resistance between each terminal.
Confirm that the throttle valve is fully closed.
Connect an ohmmeter to terminals IDL and E2.
On 1997 5VZ-FE engines only:
Insert a 0.013 in. (0.32mm) thick feeler gauge between the throttle stop screw and lever. Check for continuity between terminals VTA and E2 of the sensor. Resistance should be 0.2–5.7 kohms.
Insert a 0.021 in. (0.54mm) thick feeler gauge between the throttle stop screw and lever. Check for continuity between terminals VTA and E2 of the sensor. No continuity should be present.
On all other model years using the 5VZ-FE engine, simply verify that the throttle valve is either fully closed or fully open to obtain the indicated resistance readings.
If the resistance is not as specified, replace the TP sensor.

More info here.