Jonathan :
Found this information from http://www.evenfive.com/volvo/
Check this site , they have more tips on oxygen sensors.
And Sin had posted a fix as to torch away all the build up around the faulty sensors so as to clean it and put them back on.

"Rod
O2 sensor part fix
I found a generic 4 wire sensor from Bosch at Auto Zone. Bosch #15717. Spliced it in, reset the check engine light and it fixed the problem. It cost $49.95 vs. $200 from Volvo. I couldn't be more pleased."

PChiu

Vacuum lines... as other posts will mention, check the vacuum lines closely as I had this problem and after replacing said hoses... no problems... plus a nice boost in power as now everything is working as it should!

Good luck!

Shawn

If there are codes for both O2 sensors, like 435 and 436 then before doing anything I'd carefully look at your vacuum lines because it's rare that both would be bad at the same time. A vacuum leak trips both O2 sensor codes. If non-turbo, remove the plastic cover over the throttle body and look at the hoses going to the vacuum T there. If turbo, check the hose from the intake manifold, behind the PS pump and its outlet at a T just above the turbo air intake going into the large air intake tube. Those are the most common areas where I see cracked or deformed (from oil) vacuum lines.
Inexperienced techs will often misdiagnose those 2 codes together for both O2 sensors.

You can get cheaper O2 sensors but they are generic and ou have to wire them yourself. Stick with the Bosch for easier installation...it simply plugs in. I found one at Advance Auto parts for $130 several months back but you may be able to do a little better if you search.

Jim