I have a 2000 Legacy Gt with a used 20 engine. The long term fuel trim is reading - 15 and the #1 o2 sensor is reading 3.7 and up volts. There is not an engine light code. Is the the car running rich and should I be concerned?

Has your gas mileage gone down? Running rich will use more fuel, hurting your mileage. If it hasn't, all is probably normal.

 

GT would have the 2.5 motor, did you swap it with another 2.5, or did you put a 2.2 in there?

A 2.2 will run rich on the phase2 2.5 computer because it uses MAP to control fuel.

Yes do you mean 2.2, 2.5?