Full 6-point socket or box only (tight fit and good alignment), and light sharp taps in the tighten direction with a light hammer first to help break the corrosion up. You're not after force, but sharp vibrations. Then alternate with right/left taps for several rounds, and then no hammer and try going at it with alternating direction if necessary. Patiently worry it out. 30 minutes of penetrant soaking with tapping helps (keep the lube off the hex), as can heat differential between the part and base metal as locksmitty said. So a small chunk of dry ice for a couple minutes on the sensor only can help to shrink it away from the manifold. Worst case, the sensor body is thick but hollow, so a punch and hammer working around from the side (don't hit the manifold) can collapse it a hair and add more vibrations. Good luck!
David