The cylinder head gasket provides a seal between the engine block and the head that caps the cylinders. The gasket not only contains the pressure of controlled explosions within the cylinders but also isolates the cylinders, oil and cooling channels from each other. Head gaskets can fail due to age and miles, or as a result of overheating. Telltale signs include rough initial engine operation when cold, overheating, white exhaust smoke (not to be confused with simple engine vapor in cold weather) or the presence of oil in engine coolant. Coolant leaking into the cylinders can damage the engine and destroy the catalytic converter, at a substantial cost.