1996 Plymouth Voyager Have two codes PO123, PO121

It's on the throttle body near the throttle shaft, with a three-wire connector. With the key on, and using a multimeter backprobing the connector (I use straight pins and an alligator clip attachment to the multimeter leads), one outer wire should show 5 volts, the other continuity to ground. THe middle one should vary smoothly from about 0.2 to about 5 as you open the throttle.

check here: https://www.2carpros.com/trouble_codes/obd2