Had check engine light with code P0031, which is the upstream passenger side O2 sensor (bank 1, sensor 1). My 2007 G35x sedan has 166k km. started spraying the sensor a week prior with penetrant. ordered an NGK 29416 universal O2 sensor from Amazon.ca, along with a compact O2 socket. I cut the sensor wires from under the vehicle but found it impossible to loosen it from below. Used a deep 22mm impact socket, flex joint and extensions from above. Broke two 3/8" flex joints, man that thing would not budge! Did more internet research; I was worried I might shear the thing off.
Found a formula for penetrant which is half transmission fluid and half acetone and dripped some onto the sensor then went for a drive to get a 1/2" impact universal joint.
Torqued on the sensor with an 18" breaker bar plus 15" of pipe and it finally budged.
Sprayed more penetrant and worked it back and forth until I could unscrew it without thread damage. Once out, I spliced the new sensor wires to the original harness connector and installed it using the O2 socket. Started up, reset the code and happy to say it's fixed, all for under $60.