Which model Volvo?  Many can be replaced with Bosch "generic" parts for
a fraction of dealer prices, the only difference being the connector (if
you don't mind cutting and crimping a wire or 4).

    Bill

I have had limited success with a hot propane torch followed by my ultra
sonic bath- you've nothing to loose. If the working end of the sensor has
suffered from:

a) antifreeze (failed headgasket)
b) prolonged exposure to over rich mixture, or raw fuel (from a misfire)
c) lead (from leaded petrol used in error)
d) silicone (silicone sealer-again from use during engine work)
e) excess combustion of oil (rings, head gasket failed etc)

then you're wasting your time. If you bugger the cat through running for a
while with a failed o2 sensor then its even more costly.

Tim..