2009 CTS4 3.6L

P0430 catalyst below efficiency bank 2 comes on periodically. (There are no other codes being thrown.)

I put a data logger on it today on my drive to work and the rear sensor is certainly erratic at times. It doesn't necessarily follow the front sensor signal.

I thought the most reasonable way to test it would be to swap the sensors side to side. Then if the problem follows the sensor, it's the sensor that is bad. If the problem stays on the driver side, it's the catalytic converter that has gone bad.

However, due to the placement on the driver side, the entire pipe has to come down to access it. As I've just had twin daughters, my time for repairs is minimal and if I'm going to drop that pipe I want to only do it once.

Is there any way I can conclusively determine if it is the sensor or converter prior to removing anything?

1 Answer 1


This is by far the best video I've seen which gives two ways to tell if your catalytic converter is bad. In a nutshell, Eric the Car Guy uses two methods to diagnose a bad converter:

  • He uses a scan tool which uses live data to do just what you are looking at. He is checking to see how the downstream O2 sensor is behaving. Basically it should be fairly steady, while the upstream one is wildly all over the map.

  • Secondly, he uses an infrared no-touch thermometer to read how hot the exhaust where it goes into the cat and where it comes out. If the cat is good, your readings coming out of the cat should be much hotter exiting than going into it. If the catalyst is doing it's job, it's going to be burning the excess hydrocarbons, which will cause heat.

If you do either of these test, ensure the vehicle is completely up to temperature. If you don't, you won't get accurate readings.

I do disagree with Eric on one point of the video, that being "the catalytic converter is there to clean up NOx emissions" ... while a cat does do this, it is there also to clean up Carbon Monoxide (CO) and Hydrocarbons (HC). For this reason catalytic converters are called three way converters.

  • 1
    There is also the propane test, seeing the OP has access to pre & post O2 sensor readouts
    – Zaid
    Mar 11, 2015 at 14:23
  • i also viewed that video by eric and i believe that is very conclusive in determining the CAT condition...for the o2 sensors i believe there are videos out there which show how to test if they're good or bad. Dec 27, 2020 at 4:03

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