Check all the vacuum lines and rubber couplers/elbows. My non turbo 1998 V70 had the PO442 code. It always came back within a couple of weeks after clearing. It went away permanently after replacing both rubber vacuum elbows at the canister. My canister is under the drivers side front fender, visible from under the car. The bad rubber elbows looked OK with no cracks, however the rubber was soft and the surface was rough.
So replace any rubber elbows or couplers that dont look new. I used sections of standard vacuum hose - Volvo sells couplers and elbows.
Mike
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