1996 Mazda 626 Oil Leak

I am driving my brother's 1996 Mazda 626. Oil is dripping from a sensor installed in the engine block just above the oil filter. it appears the sensor has a collar on it that is meant to grab with a wrench to tighten or loosen. It is a very tight spot (under the air intake behind the block). From under the jacked up car I barely managed to get a small pair of pliers on it to try to tighen it and/or remove it to examine it. I had only a little leverage, but I could not budge it. I do not have a repair manual for this car, and I am not even sure I am doing the right thing. I don't even know what sensor I am trying to remove. Request any help in how best to remove, or even the name of the sensor. Thanks, Sean