Professional Corner

Diagnostic - Failure

Principal Reasons for failure
Incorrect fuel-air ratio or engine misfire

Catalytic converters require the right mixture of exhaust gases and high temperature to function correctly. Faulty carburetors or injection systems can produce over-rich exhaust gases. Incorrect timing, bump-starting or running the fuel tank completely empty can result in unburned fuel being fed into the catalytic converter. Temperatures of around 1,000ºC. can melt the substrate ("melt-down") leading to a fall in the rate of conversions and, ultimately, to total catalytic converter failure with severe restriction of the exhaust flow. This condition can be caused by engine faults or engine management problems.

Faulty Lambda sensor

If the Lambda sensor is faulty, the air-fuel mixture used in the combustion is not correct and the exhaust gazes passing through the converter may be incorrect thereby reducing conversion efficiency.

Lead contamination

Only unleaded fuel may be used with a catalytic converter.

The precious metal catalysts are applied as a thin coating on the core inside the catalytic converter. Lead forms a thin film which prevents contact between this coating and the gases, which prevents the catalytic converter from converting the harmful exhaust gases. It takes only one or two tanks of leaded fuel to permanently deactivate the catalytic converter.

Petrol tanks on all new cars are fitted with narrow pipes for unleaded petrol which prevent the wider nozzles of leaded pumps being inserted.

Physical damage

If the stainless steel casing of the catalytic converter is subject to hard knocks or excessive vibration, internal damage may result making the catalytic converter inoperable because of a broken monolith.