It requires the removal of the intake manifold, the separator box is under it. At least a 5 hour DIY job, for the first timer.
If you have always used synthetic engine oil, the separator box should be relatively clean. The things to watch are the vacuum line that comes out of the intake manifold behind the PS pump (if it is cracked and appears to be leaking, then replace all of the lines when you take the manifold off), and pull out the oil dipstick and check for light suction at idle. Any pressure there will mean that you need to replace the PCV system.

There is a write up on Volvospeed for Knock sensor replacement, which shows what is involved.

Klaus
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Just driving a 1998 V70R :)