Any condition that causes abnormally high levels of unburned hydrocarbons (raw or partially burnt fuel or oils) to reach the converter will tend to significantly elevate its temperature, bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and severe exhaust restriction. These conditions include failure of the upstream components of the exhaust system (manifold/header assembly and associated clamps susceptible to rust/corrosion and/or fatigue e.g. the exhaust manifold splintering after repeated heat cycling), ignition system e.g. coil packs and/or primary ignition components (e.g. distributor cap, wires, ignition coil and spark plugs) and/or damaged fuel system components (fuel injectors, fuel pressure regulator, and associated sensors). Oil and/or coolant leaks, perhaps caused by a head gasket leak, can also cause high unburned hydrocarbons. Since 2006 (in the United States), 10% ethanol has been added to gasoline. Ethanol fuel can dissolve large amounts of water, potentially carrying corrosive ions and acidic compounds to fuel system components. Ethanol itself can also be digested by acetobacter in presence of water, generating corrosive acetic acid. These factors can result in corrosion of fuel system components and abnormally high air fuel ratio.