[XR400R]

PO174 is an Isuzu code indicating a lean mixture condition on bank 2, which is drivers side bank. Due to the importance of the O2 sensor in the closed-loop feedback fuel control system, it is entirely possible the O2 sensor is the problem. Since you have the sensor, why not change it & see?

39K miles on an '02? Surely that is not the total milage on the veh? In my limited experience (40 yrs shade-tree wrenching - long before O2 sensors existed), it is very rare for O2 sensor to fail after only 39K miles. Unless they've been poisoned, hit or otherwise abused. I've had em last 250K miles. I usually change em at 100k whether the need it or not. A fresh O2 sensor will frequently improve runability and fuel milage, in my experience. Just my opinion anyhow.