1995 Dodge Caravan emissions

21 - Oxygen sensor signal, neither rich or lean detected.
51 - Air/fuel at limit.

Fault codes never say to replaced parts or that they're defective. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. A sensor referenced in a fault code is only the cause of that code about half of the time.

In this case neither code refers to the throttle position sensor. A mechanic would start by using a scanner to view the signal voltage coming from the oxygen sensor. Without a scanner, you can inspect the wiring to the oxygen sensor but beyond that, you'll probably just have to replace it and see if that solves the codes. Throwing parts at a problem isn't the way we like to diagnose things, but sometimes there's no better alternative.