1993 Buick Lesabre Accelerated RPM after engine reaches ope

We do most of the repairs to our vehicles. We have a 1993 Buick LeSabre with only 20,625 miles. It is only a weekend car. Recentlly we noticed that after the vehicle reaches normal operating temperature the RPM increases suddenly to about 1,700 RPM. This is very dangerous. After using the diagnostic scanner we noticed code 21 which entails a high voltage output in the Throttle Body Positioning sensor. We would like someone to advice us what will be the next step to properly repair this vehicle. The repair manual calls for the replacement of the TBP sensor. The repair manual is not clear on whether the engine should be running or not when measuring the TBP voltage from the electric harness. Could it be the temperature sensor responsible for this problem? This was the case with our 1993 Saturn SL2 (after the temperature sensor was replaced the sudden acceleration problem was solved). We would appreciate very much your input. Thank you.

Jim