Sometimes to diagnose a cylinder head gasket fault can be difficult.
However, there is a block/head gasket checker that can be got.When a cylinder head gasket blows it normally lets in combustion gases into the cooling system(which should never happen).The head gasket is the main gasket that goes between the engine block and cylinder head.In the photo are the head gasket checker tool and a blue chemical that is put into it.The checker is then put over the expansion bottle and with the cap of gases are drawn up into the device.
If combustion gases are present the chemical turns from blue to yellow.
