special tool needed to replace O2 sensor?

You can't use a socket, unless you cut the wire.

They make a special offset box wrench for it; but you may be able to use an open wrench.

You'll def> Can anyone tell me if I need a special "split" or "slit" socket (22 mm)

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I have replaced my O2 sensors on my 97 328. I use a 22mm or 7/8 inch "Flare Nut Wrench". It has a notch cut in it that will fit over the wire, and grabs a majority of the 6 corners on the sensor nut. You can get one at Home Depot, or auto part stores. I think mine came from Schucks (Krogers). Sometimes the bend on the wrench end isn't quite enough to fit well, but a vise and some heat, and you can re-bend the wrench end enough to fit easy.