With 4L60 Transmission

Circuit Description
The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to the throttle opening. Signal voltage will vary from about 0.5 volt (0%) at idle to about 5 volts (100%) at wide open throttle. When there is a Code 21, EPR is shut "OFF."
Code 21 means that the ECM has seen the following:
^ High voltage at ECM terminal "A2".
^ For a time in excess of 2 minutes.
^ Engine speed less than 1120 rpm.

Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Confirms Code 21 and that fault is present.
2. Checks for 5 volt reference at TPS harness connector and separates an electrical circuit problem from a faulty TPS. If the circuit is OK, normal voltage reading will be about 5 volts.
3. Check to see if low reference voltage is due to an open wire or the ECM.