P0420 diagnostic trouble code is a “Catalyst Efficiency Below Normal” (Bank One)

This applies to vehicles equipped with OBD2 only. It basically means that your computer has detected that the Catalytic converter has failed on bank one. (Bank one is the side that the number one cylinder is on. Bank two is the other side.)  After the computer sees that this test fails so many time, will will store this P0420 code in the computer and light the check engine light or MIL light.

Diagnosing P0420 catalyst efficiency code

To be able to diagnose this p0420 converter efficiency code, we need to have a good understanding of how this system works. From all of my years going to class the easiest way I can describe it to you is that the catalytic converter stores oxygen. When it goes bad, it no longer has the ability to store oxygen and the computer will flag this P0420 code. On a computer controlled OBD2 vehicle, there will be one oxygen sensor before the cat and one after the cat. When the engine is running and is fully warmed up, the computer compares the two o2 oxygen sensor signals for a predetermined amount of time. If the converter is storing oxygen, then the rear oxygen sensor signal will produce a much different signal back to the computer than the front oxygen sensor. This is good. This is what the computer is looking for. This means the catalytic converter is doing its job (storing oxygen). When the computer sees that the two oxygen sensor signals are the same, this indicates that the catalytic converter is no longer storing oxygen like it should. The computer will then light your check engine light or malfunction indicator lamp and store the fault code P0420.

How do I know if my converter is bad?

If you get this P0420 diagnostic trouble code and you want to do some of your own testing, there are a number of ways to check the catalytic converter. One good way to check your converter to see if it is “lighting off”. This means that the converter is burning up the emissions on the inside. Remember, the hotter the converter gets, the cleaner it will run. The converter should be about 100 degrees hotter going out that coming in. You can use an infrared temp gun to do this test. Raytek makes a pretty good temp gun for a reasonable cost.  You are going to want to be sure that the engine is full warmed up and run the engine at 200 rpm for about 1 to 2 minutes to prior to running this test.

Another thing that can happen is the converter can plug up and cause a false P0420 code in the computer. This usually happens when the engine has had a misfiring or rough running in the past and the customer continues to drive the vehicle for a long period of time. The converter will in fact get so hot that to the point that it will actually melt the brick inside into a ball. When it cools back down, it can tend to cause a low power issue and will eventually flag a P0420 code. You can use an exhaust back pressure tester to check for this condition.  Waekon makes a good exhaust back pressure tester.  I personally have this tool and would highly recommend it.

False P0420

Why would the computer flag this code if the converter is good? What other possible causes would there be that would cause this p0420 code in the computer? Besides the two possible causes I mentioned above, there are several things to take into consideration. Exhaust leaks, bad oxygen sensor or sensors, lack of power to the oxygen sensor, bad PCM (computer) or even a PCM that needs to be updated or re flashed can all cause your P0420 catalytic efficiency code. If you have a P0420 code in the computer along with an oxygen sensor code, be sure to diagnose and fix the oxygen sensor code first before going after the p0420 code.

Here is an interesting site that talks about this same P0420 diagnostic trouble code.  Below is a video explaining a bit about the catalytic converter and how it works.


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