Checks for combustion leaks caused by bad head gaskets, cracked blocks, and cracked or warped heads. Fluid changes from blue to yellow when exposed to combustion gases from the radiator. Connect the hose to any vehicle vacuum line. Or, on tools made after June 2002, use the red adapter with a hand vacuum pump. Place tapered end of tool into radiator opening. With the engine running, the vehicle vacuum or the vacuum pump draws vapors through the indicating fluid for testing.