What could cause a cat to fail?

Normal aging

A catalytic converter is subjected to high temperatures (in excess of 800°C) and mechanical stresses (vibration) during normal vehicle operation. This can cause the washcoat and precious-metal coating of the substrate to degrade over time. Nowadays, one generally assumes an average service life for a cat of 80,000 to 100,000 km (50,000 to 60,000-odd miles).

Mechanical damage

  • Damage caused through bottoming-out: If the catalytic converter bottoms out when a car is driven over some kind of obstacle (a kerb, for example), the mechanical load is often so high that the casing of the cat is dented, in some cases badly. This can break the catalyst support (aka substrate or monolith) housed inside the casing. The loose, broken bits inside the casing then fly around and cause rattling noises.
  • Pipe break: If the pipes that connect the cat to the rest of the exhaust system break, the cause is usually one or more defective or missing mountings or a worn-out engine mount. As a result, the mechanical stresses on the pipe joints are far higher than they should be. The pipes usually break in tight bends or in the immediate vicinity of welds.

Catalyst poisoning

If an engine burns too much oil, the additives contained in the oil can become deposited on the surface of the catalyst support and completely seal it. The exhaust gases then no longer come into contact with the precious metals and the catalytic converter no longer functions. Frequent short trips can also poison the catalyst, i.e. the catalytic converter. In this case, however, the cat can often be regenerated by taking the car for a decent trip down the motorway. The old danger of poisoning the catalyst by filling up with leaded petrol no longer really exists, unless you happen to drive to some foreign country where leaded petrol is still available. In this case, you must take special care to ensure you only fill up with unleaded fuel (bleifrei, sans plomb, sin plomo, senza piombo).

Melting of the catalyst support

Faults in the ignition or fuel management system can lead to unburnt air-fuel mixture finding its way to the catalytic converter and burning up on the surface of the catalyst support (substrate). This can cause temperatures in the cat to exceed 1000°C and to the catalyst support being destroyed.

Last update on 2013-07-05 by Dominique Winkler.

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