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.
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.
This is great info, but what could cause a false positive?
I’ve got an interesting situation. On an 08 Hyundai Accent, with the cat reading 550C (from the scan tool), (1 bank) S1 and S2 look similar until the Evap purge turns on. When the evap purge is operating, S2 looks like the “good” example above. As soon as it turns off, it goes back to the 2 voltages being almost the same.
Fuel trims look fine (long +6.3%, short -3% to +3%), and there aren’t any performance issues.
I wish to understand what equipment I need to buy to test catalytic converters. I have legal ownership of a large on going number of them. I am willing to engage a consultant to help design a proper testing operation.