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. The block tester kit includes: - 1 bottle of Block tester test fluid. - Universal Block Tester - Bulb - Complete Tester - Complete Instructions Block Test Fluid spare is also available. 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.