If you've ever tried removing one, the phrase ‘Lambda sensor’ will surely induce a sigh or, if it was anywhere near as immovable as this cast iron manifold, a shiver will have run down your spine. No matter how you well you get that socket or spanner on there, that thing is so stuck in you risk rounding the hex. Don't risk it, IReOx it! Make up a solution of IReOx and drop that manifold/exhaust in there for a few days. The IReOx will not only remove the rust from your manifold/exhaust surface, it will get to those area’s inside the primary tubes, that you simply cant get a brush or blaster!
It will make its way into the threads, removing any corrosion and allowing you to unbolt the sensor. It gets better, it works on all of the manifold flange studs too, no more broken studs! nb: Unfortunately IReOx can’t fix an over-tightened sensor or stop your spanner rounding off the hex, but it gives you a head start on getting the blighter out!