They fail often on '99-01 Volvo cars, very very common. It's the electronic throttle module but the unit will usually have a code for "internal fault" which means "REPLACE". Replacing the unit is the only solution then. Sometimes it sets the light and you might not notice driveabilty problems right away but eventually it'll act up. You also can't replace the part yourself as the new throttle unit won't work until software in installed by Volvos Vadis cart (dealer only). Even if you or another shop replaces the part, the car won't run until SW is installed so you'ld then need to have it towed to a dealer. Might seem strange, unfair, uncool, etc BUT true.