Sensors In Transmission

I recently bought this 94 Seville. Only 75K. Beautiful car, too good to be true. I have replaced the A/C compressor and am getting a "Service Engine Soon" light. My mechanic told me it was a transmission code. Calling an "expert" he told me it could be an internal sensor code which could require an entire rebuild. I am having the fluid and filters changed next week. I do not notice ANY indication of a transmission
problem. The fluid on the stick looks ok. Could this assessment be legit?
I need your help!!

Hmmm, I'd want to know EXACTLY what sensor it is! There are certain ones on the 4T80-E transmission that can be replaced without removing it! See if he can give you the code that came up for the sensor.

[Thanx CD. The code is something like PL3P. The mech showed me.
It deals with the torque-converter. He said if I leave it tomorrow
he will research it and tell me my options.

The code should be something like P0xx with two more numbers for the powertrain codes.
If it's for the Torque Convertor Clutch (TCC), there is an updated PROM chip for the computer that can cure this problem. This car has the Northstar, right? I did this on my old 1994 Deville Concours and it cured the TCC problem and only cost around $35.
The code that is for the TCC is P039.