The emissions light (also called a MIL light or "check engine" light) is not
normally a critical alert, but read your owner's manual for details. A
common cause is a loose gas cap (although it may not feel loose), but it
could be lots of things. Any service department and most auto parts chains
can read the codes and determine what sort of thing is going wrong. But for
the warranty (assuming it isn't the gas cap) you need a Volvo garage.

In general, when the emissions light comes on, if you don't notice anything
else the car is okay to drive around town or gently on the freeway. If the
fuel consumption is okay also, it can be driven normally.

Mike