Oxygen sensors?

Most of the time with the proper tool, factory installed oxygen
sensors break loose with little difficulty, since the factory
uses the proper anti-seize compound on them.  Sometimes they can
be a bit stiff to thread out because stuff has built up on the
threads, but they generally come out.  It's when some backyard
bozo replaced them already and cheaped-out by not using the
proper anti-seize, that you may run across difficulty.

Some replacement oxygen sensors come with a small package of
anti-seize compound in the box.  Some dishonest auto parts store
clerks steal them.  Check before you leave the store.