Block Test Fluid for Block Tester ========================= Now you can detect exhaust gases in the radiator header tank and check for gasket or block damage, cracked heads or cylinder warp. Block tester is a blue coloured reagent that turns yellow when exhaust gases are present. This simple diagnosis can save you £s. Suitable for petrol and diesel. Buy Full Block Tester Kit for Petrol and Diesels Cylinder Heads / Gaskets here . See the How Do I Flush and Refill My Cooling System?  and  How To Quick Check Head Gasket With Block Tester articles in the Technical Articles section.