Recently I changed the spark-plugs and out of the blue, six codes pop up. The seventh was undecipherable "manufacturer code". It was evaporation sensor, idle sensor, front and rear oxygen sensor heat and speed sensor.
I checked with instrument separate sensors and they according to specs are fine. Attempt to reset the codes with standard ODBII failed. Finally, today i borrowed Volvo computer and for oxygen and evaporation sensors diagnose was "weak signal" poor or to that effect. Could anybody think of plausible action before I go to a dealership wich might rip me off without guarantee that faults will be repaired.

any update?

After tedious search, turned out to be a burnt fuse f the exhaust system in main fuse box. Very simple, not a repair really. The trick is to know where to look first... And how to look.

Here's my first hunch,

Look to see if you may have a ground wire off or broken.
Especially at the intake manifold.