First replace the O-rings since it's cheaper, simpler and the cheaper Nissens aftermarket matrix has less and bigger diameter (to have same total flow) parallel flow tubes than the original and has therefore less coolant-to-heater surface area.
Hella/Behr one is same as OEM but costs more than twice the price so just replace O rings and hope that's done with it.
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2002 V70 D5
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