The bevel gear is similar to the transfer gear in 4wd trucks, except the bevel gear is always engaged. It sends the power to the rear driveshaft. The bevel gear fluid needs to be replaced every 50K miles, less if you make use of the awd system a lot - live in snow/ice climate or on dirt roads.
There is a procedure that involves drilling a hole in the bevel gear case to allow the fluid to drain so it can be easily flushed. The write up is over on Volvospeed.com, do a search there.
On the other topic. You blindly took your car to a dealer and believe him? Did he tell you the code number? It should be on the written report.
You have an 8 year old car, the elbows on the vacuum lines entering the intake manifold could be cracking and letting in unmetered air. This will cause the ECU to run rich and the front O2 sensor gets confused and sets a code because it can not react fast enough to the changing fuel conditions.
Or, you really do have a failing O2 sensor. In that case, replace the front O2 sensor. The rear sensor usually lasts longer than the front.
When the O2 sensor has failed for a long time, a second code is generated warning of possible damage to the cat converter. This is just a warning, but the dealer can tell you that the cat is damaged and point to the message. Seldom does the cat fail unless the engine gets neglected for a very long time.
Klaus
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Volvoless. Sold the R. There is now a void...
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