How to care for your vehicle’s cat

A catalytic converter can cost as much as R5000, so looking after the one on your car makes good financial sense.

A catalytic converter can cost as much as R5000, so looking after the one on your car makes good financial sense.

Published Mar 18, 2011

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I recently wrote about catalytic converters and this created a lot of interest. In fact, a friend in England phoned me to say that he had renewed the two on his BMW 740i - at about R5000 each. All the more reason for having any car you are thinking of buying thoroughly and expertly checked, including the cat(s), before parting with your cash.

If the car has a cat check indicator light, make certain that it stays on for only a couple of seconds. If it remains on, be suspicious. If the car has a pre-cat test port, a well-equipped technician will be able to check the gases before and after entering the cat.

Cats get very hot in operation which is why they have heat shields. Normal operating temperature will be 350 to 400 degrees, but when unburned fuel enters due to an over-rich mixture, temperature soars and meltdown occurs.

A simpler test requires an infra-red thermometer; when the cat is okay, the temperature at the outlet should be higher than that at the inlet.

In theory, the cat should last as long as the car itself and if treated right this can be achieved. But, unfortunately there are inherent dangers. Fuelling faults, especially caused by a rich mixture, will eventually cause a meltdown. Some small-capacity engines have suffered from cat trouble because they come with a richer mixture to ensure smooth running at low revs.

Conversely, larger-capacity engines used mainly for short trips may not give the cat sufficient time to get warm. The same goes for diesels. Visual checks are fine but sometimes the heat shield gets in the way. External damage and signs of overheating are bad news.

Regular servicing at the specified intervals is important for cat-equipped vehicles. Badly worn or incorrect spark plugs could lead to unburned fuel reaching the cat via the exhaust and goodbye to a couple of thousand rand. The same thing happens if you bump-start the car.

Renew the oxygen sensor at or before the stated intervals as failure here could wreck the cat. The sensor may be checked by connecting a voltmeter to its output and checking the voltage against the voltage specified in the manual.

OK, oxygen (lambda) sensors are not cheap and have a finite life - usually about 80 000km - but you cannot afford to ignore renewal recommendations. And, anyway, when the sensor is past its best petrol consumption will be higher as will exhaust emissions. General maintenance and servicing is now more important than ever. If the engine is burning oil or there is a head gasket leak, the cat will suffer.

The honeycomb brick inside the cat is tough enough to do its job but, like granny’s favourite china, does not enjoy sudden shocks. So watch out for speed bumps (some are almost invisible!) and go over them slowly. The same goes for potholes. Avoid driving through fords or other deep water because this may eventually cause failure.

It is essential to thoroughly check all other possible faults before renewing a cat. Imagine spending cash on a new one only to find the oxygen sensor was faulty and the new cat goes the same way.

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