A clever yet simple to use device used to detect leaking combustion gases that may find their way into the cooling system when a head gasket blows or block cracks once fitted to the running engines coolant filler neck the rubber bulb is used to suck the gasses that collect in the top of the filler bottle through the fluid in the tester if there is co or co2 in the gases it will turn the fluid green indicating a blown head gasket or similar issue