Hi

It is called a valve and the part number is 3298596. The best solution is to remove the two silver values and cap them with 3/4" caps available in most hardware stores. The air lines can be plugged also.

My car runs just fine with all those pieces removed.

If the plastic valve broke on side closest to the two large check valves, then one of the check valves is bad. The plastic valve with get the full heat from the exhaust if one of the check valve sticks in the open position as it did in my car. I think the check valves are only available used.

Look on the bottom of the page on this link.

http://www.volvoclub.org.uk/faq/EngineSensors.html