Had the same on my sisters S80 non turbo couple months ago ... changing the coolant sensor (about £20 - get one with the plug on!) fixed it nicely ... mind you the coolant was brown so I did a full flush and popped a thermostat in as well.
Whole job was under £50 in parts and it now runs properly ... Her's would also cool down if the heaters were opened up for a bit but then still went high.
Iain
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S70 T5 1999 (now written off grrr)
S80 2.5T LPG 2004
XC90 D5 2005 (2006)
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