Just my word on O2 sensors........I have OVER two hundred trucks (CAT NISSAN MITSU's) at my various customers that I service with the Nissan K21/K25 Engine....some with 14,000 Hrs PLUS and have NEVER replaced an O2 Sensor...it is more than likely set because the connector was unplugged when it was running or when your repairs were made some how the wiring pigtail to the sensor was laying on the exhaust and melted the wiring......