2009 S80 T6, Not sure what I could have done but... I took half the top of the engine off to get to the bank1 O2 sensor, replaced it, put it all back together and now my brakes are stiff and barely stop the car when I stomp on them, seems like I have lost the power brakes. I took all the plastic crap back off the top of the motor and checked all the wiring connections, everything looks good and no check engine light or vehicle messages, anyone got a clue???

 

Sounds like you have lost the vacuum going into the brake booster. That could be due to the lines or your vacuum pump is not engaging. Does it get better with the engine at speed?

 

Thanks, it was the hose from the booster, I cut it taking out the stupid false firewall, all fixed now