Road Damage: This is external damage caused to the catalytic converter by hitting solid objects in the road. i.e., speed bumps, large rocks etc.

Plugged or contaminated catalytic converters are caused by using the wrong sort of fuel in your car. Using leaded or lead replacement fuel will plug up the monolith and cause it to stop working. A similar thing will happen if fuel additives are used that are not suitable for use with a catalytic converter.

Oil Fouled: This is caused by oil getting into the exhaust system and contaminating the catalytic converter. The most likely cause is a leaking gasket.

Melted/Broken Substrate: A monolith is usually broken when it is impacted by an object or when it suffers a sudden change in temperature. If the cat suffers road damage (See section on "Road Damage") the monolith inside can be cracked due to it being crushed by the movement of the steel can.

The use of exhaust paste before the cat can also cause the monolith to break. When the exhaust paste has hardened small pellets can break away and shoot into the cat. The monolith will gradually be destroyed by these pellets and break down. The cat can also be destroyed by excessive engine vibration.

A rapid change of temperature can also break the monolith. The monolith can be melted when unburned fuel is injected into the cat. This can be caused by the car being tow or bump started, or if the car's engine requires several turns before firing. More information on the causes of melting monoliths can be found in the section on "Overheating".

Overheating: There are many problems that can cause a cat to overheat or fail. The most common cause is unburned fuel entering the cat. Also faulty spark plugs and leads will cause the engine to misfire and ruin the converter. The cat will also be ruined if the distributor timing is out.

Oxygen Sensor: An oxygen sensor collects and sends information to the Electronic Control Unit. This is used to control the air/fuel mix. If the oxygen sensor is faulty, or it has been contaminated with silicone from anti-freeze or sealant, the air/fuel mixture will not be corrected and excess fuel will enter the cat, causing it to overheat.

Fuel Injection Systems: If a fuel injector is leaking internally or dribbling fuel into the engine the converter may be damaged. The excess fuel will enter the exhaust system and cause the cat to overheat. Due to the varying fuel injector systems the proper manual should always be consulted when trying to diagnose a problem.

Map Sensors tell the Electronic Control Unit the load on the engine and the amount of air entering it. If this sensor fails it causes a rich condition in the engine which can overheat the cat and cause it to fail.

Carburettor Systems: A worn or defective carburettor can cause a cat to overheat. Problems such as improper float or air/fuel mix adjustments and worn metering rods can damage the cat. The choke system also needs to be operating properly so the right level of fuel enters the system. If too much fuel enters the system the cat will overheat and eventually fail.