Trouble After Timing Belt Change
 

I have a 2000 S80 non turbo. Last week I had the timing belt changed along with the water pump. The car was running relatively good prior to the change. It had a slight engine knock that remained a bit of a mystery. Now after the timing belt change the engine knock is signifcantly worse. The check engine light instead of just being on steady now blinks at me. I have no power none. Push the peddle to floor and nothing happens. I took the car back. I was told and they changed the spark plugs and something about the evap system was fixed. They found water in the spark plugs. I was wondering if that would be bad gas? I did fill up just after timing belt change. No change in performance after changing the plugs out car still has no power.

I know at least one of the O2 sensors needs to be replaced. They are telling me now that it is O2 sensor. I am wondering if the poor performance is a result of an O2 sensor or something has gone wrong when changing the timing belt. The other thing is that when stopped the check engine light stops blinking. Only when gas is on does it blink.

The question is, could one O2 sensor cause this much trouble and if so can I replace them with a Bosh sensor instead of spending 300 dollars on a Volvo one?

Any assistance is muchly appreciated