Throwing 6 different codes
 

Check engine light has been on for awhile, I cleared the codes a week ago (8 at the time) and today when I pulled them again they are:

P0134
P0138
P0477
P0422
P0432
P0455

Where should I start looking? Its tough to find good information on all of these codes, I'm tempted to dump a can of cataclean in the tank to see if that helps...

OBD2 codes
 

0134- front heated oxygen sensor
0138-rear heated oxygen sensor
0422- 3-way cat. converter efficiency
0455- leak diagnostic
check the numbers on the others; they didn't cross over

I believe the 455 has been on and off for awhile but the cat and the o2 sensor codes just crept up on me recently. it seems odd that 2 o2 sensors and 2 cats would all go at the same time, wondering if there is something else influencing them?

I do not have vadis running at home at the moment but if they are both on the same bank(O2 sensors) the front one might have gone bad and taken the rear one out. I would replace the bad O2's and take it from there. The cat code is a code that comes up when it is running to rich or to lean. It might not come back after replacing the O2's.

OBD2 codes
 

I think most of the different sensors are 5 volts similar to industrial automation. I would be temped to check the voltage first. I would defer to the volvo tech though. I was a boat mechanic and they were still using carbs and distributors until recently and no emission equipment to speak of.