A test conducted on Stage II vapor recovery systems to determine if liquid may be collecting in low places in the piping, and thus blocking the flow of vapor back to the storage tank. The blockage test is prescribed by the PEI Recommended Practice for Installation and Testing of Vapor Recovery Systems at Vehicle Fueling Sites, PEI/RP300.

The test involves passing nitrogen through the vapor-return system. Obstructions in the piping, such as those caused by an accumulation of liquid at low points, will impede the flow of nitrogen and indicate the presence of the obstructions.