It regulates the air going in and out of the fuel tank and evaporative fuel tank in trunk. Mine was clogged from debris and failing filters...I cleaned it out well and re-installed. Works fine without the filters. If you install incorrectly fuel pressure will build up (or be sucked out of) fuel tank and the pressure can dent your tank or damage your evaporative fuel tank. Not good.

If you find a source for a new one let me know...would still like to get my hands on one - or the filters....which are item number 1074710042.