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Catalytic converters don't last forever as some may think. They deteriorate with age and can be destroyed through running lean and rich mixtures for excessive periods of time. As we know from my other articles that if the engine management system fails to adopt closed loop control, the system will run rich and any miss-firing will have the same effect. The converter will melt down and the catalyst will break up and then block the exhaust system. Before starting the test, warm up the engine for a few minutes above 2,500RPM. This ensures the converter is hot as it only works correctly when hot, approx 400oC.
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An easy test is to give the converter a good old thump with your hand or foot. If you hear a rattle coming from the exhaust system then chances are what you are hearing is the catalyst pieces rattling in the pipes.
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If you have a vacuum gauge, connect it to a manifold vacuum source on the engine. Rev the engine to 2,500RPM and hold. The vacuum should drop momentarily then return to just below normal vacuum. If the vacuum does not respond in this way, you may have a blocked exhaust, caused once again by pieces from the catalyst.
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A lazer temp gun can be utilized to check exhaust temperatures before and after the converter. Raise engine speed to 2,500RPM and block the throttle. Check the temp just before the converter and just after. The outlet side should be hotter than the inlet by at least 20oC. If not then replace the converter.
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Lastly, your mechanic can use an exhaust gas analyser to check the CO and HC readings. The mechanic will artificially enrich the mixture to see if the converter can hold the CO & HC down to normal. If these pollutants increase then the converter may be faulty.
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