Looking after your car’s exhaust cat

The most basic test is to listen for rattles which could mean the honeycomb inside is broken.

The most basic test is to listen for rattles which could mean the honeycomb inside is broken.

Published Jul 28, 2011


Catalytic converters are out of sight and out of mind and, although today they are almost as much a part of motoring as wheels and tyres, remain a mystery to many people.

In fact, if you were to ask, some drivers would not know if their vehicle had a cat or not.

They can be horrifyingly expensive – perhaps costing the best part of R5000 - and, as some cars have two or even four, renewal of them all can knock the budget for six.

Therefore, it is as well to know a few simple rules to keep it (or them) intact and also to know how to check if buying a used car.

The most basic of all tests is to listen for rattles which, if present, mean that the ceramic or metallic honeycomb inside has broken up. Look also for external damage and discolouration caused by overheating.

It’s a good idea to have a test done by a reputable tester before parting with your cash.

A well-equipped workshop may have facilities to measure the gases entering and leaving the cat, or use an infrared thermometer to compare inlet and outlet temperatures. The latter should be the higher.

If testing proves the cat faulty, do your homework. Find out the price of a new one so that you can perhaps get a reduction on the asking price of the car you are thinking of buying.

A full service history is good when buying any used car and because certain items should be renewed at or before stipulated mileages, examine the history carefully.

For example, the lambda sensor should be renewed every 80 000km or so. Some use a heating element to help it rapidly reach optimum temperature.

While an unheated sensor may take up to a couple of minutes to operate properly, the heated type achieves this in around 15sec. The bad news is that rapid heating takes its toll and some sensors fail in as few as 40 000km. Have a stiff whisky and check the price of a new one.

The lambda sensor is basically a tiny battery that supplies a voltage signal based on the differential between oxygen content in exhaust gas and oxygen content of the ambient air. For a pre-cat exhaust gas test connect a voltmeter to its output wire and monitor voltage. If higher than specified, the mixture is rich.

Here are a few basic tips to prevent damage to the cat. Bump-starting should never be attempted. Ignore this and unburned fuel may enter it and cause damage. Take speed bumps gently and slowly or you could easily fracture the honeycomb.

Misfires can damage cats, so if your car develops one, do not ignore it. Keep to the manufacturer’s service schedule for plug and filter changes. Ignoring plug changes may allow unburned fuel to enter the cat where it will ignite and cause damage.

Other dangers include incorrect ignition timing, and an engine that burns oil. In other words, the engine must be in a generally good state of tune to give the cat a chance of survival. Running too rich or too lean is bad news, so if in doubt, seek expert advice. - Star Motoring

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