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Catalytic Converter Efficiency Testing

Tools recommended for Catalytic Converter diagnosis include an oscilloscope/graphing multimeter, infrared thermometer, scantool and gas analyser.

Below are oscilloscope traces of pre and post cat oxygen sensors with a good and bad catalytic converter. This signal can also be viewed by graphing the sensor outputs in current data on the scantool.

For the GOOD CATALYST you can see B1S2 has little activity showing the cat to be cleaning up the mixtures, as opposed to the BAD CATALYST which has similar mixtures before and after the cat, therefore the cat is not working.

Cat-Conv-Effic

NOTE - The converter has to be at operating temperature for the test results to be valid. Cold converters will appear not to be working. Older vehicles will have a temp difference of approx. 20-50°C between the inlet and outlet, the harder the cat works, the greater the temp difference. Disconnecting a spark plug lead will cause richer mixtures, therefore the cat temp will rise if it’s working correctly.

Newer vehicles will require less activity from the cat to clean the mixtures, therefore the temp difference will not be as noticeable, this is why testing the temp is not as reliable on newer vehicles.


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