Volvo: P0136 and P0080 codes, o2 sensors, scangauge ii

Question
1996 850 GLT (non-turbo), 5spd man, 85.5Kmi

Check engine light on, ScanGauge II shows P0136 (rear O2) and P0080 (Exh valve cntl crkt hi), and 'Loop open'.  These bits never replaced before.  I'm an EE so could scope lines if necessary.

Would the P0080 code occur due to a bad rear O2 sensor?  I don't see rear O2 sensors listed on the web; is this because it should be done by dealer or do they rarely fail?  Should front be replaced at same time?  Assuming I buy the O2 tool, are both accessible to the home mechanic?

Answer
Don,
The general rule of thumb is to replace both HO2S. By them from the dealer as they should also come with a short wire harness. These are the latest HO2S, they are the newest replacements. Could someone at home do the job? Yes it will be a bit trying you need good access under the car.
I have no information on the code PO080.