I recently got an P0134 and replaced the upstream O2 sensor, cleared the codes and the light popped back on the next day, same code. The old O2 sensor was pretty black, and I've been getting poor fuel mileage so its obviously running rich. However I have no idea where to start looking next, either fuel injector or fuel pressure regulator? and how you check those? (it just was repaired for a misfire the week prior).

 

What MOTOR ???

 

2.5L V6 in a '97....my bad should have said that in the first post

 

So the exact same code came back ??? examine the wires and plug, check for steady voltage on one pin and a changing voltage on the other