Code PO420 catalytic converter

P0422 - Main catalyst efficiency below threshold (bank 1).

There is a sensor after the catalytic converters on all cars starting with 1996 models. Once the front sensor reaches 600 degrees, it starts generating a signal voltage that switches between roughly 0.2 volts and 0.8 volts. Those readings correspond to "rich and "lean", and the switching rate is about twice per second. When it is lean, excess unburned oxygen is stored in the catalyst. When it is rich, the excess unburned gas is mixed with that stored oxygen, then it is burned.

The "downstream" oxygen sensor looks at what is coming out of the converter. Under normal conditions it might gradually switch between "rich" and "lean" perhaps once every minute or two. That is proof the catalyst is doing its job. When that material becomes clogged or coated with contaminants, it cannot work properly, so the output starts to look like what is going in. The switching rate of the downstream sensor increases to, ... Lets say once every fifteen seconds. The point is the switching rate increases as the converter loses its efficiency at cleaning up the polluting gases. At the extreme condition when no change takes place in the composition of the exhaust gas as it goes through the converter, the front and rear oxygen sensors will both see exactly the same thing, and their switching rates will be exactly the same. At some point before it gets that bad, the engine computer will figure out that the converter is not doing its job. That is when it sets the "catalytic converter efficiency" code. It is simply looking at how fast the rear sensor is switching.

There are about 2,000 defects the engine computer can detect. About half of them refer to things that could adversely affect emissions. Those are the codes that turn on the check engine light. In this case spark timing and fuel metering are still correct so the engine is developing the least emissions possible. It is that little bit of pollutants we cannot help but develop that are not getting cleaned up in the catalytic converter.

It is important to understand two properly-working oxygen sensors are required for the computer to detect an efficiency problem and set this code. Replacing the sensors is not going to fix the converter.

There is a real backyard trick to clean the converter long enough for it to pass an emissions inspection. It may even work for a few months at a time. I did it on a 2000 Grand Cherokee every five or six months for a couple of years and then I sold the vehicle. It really does work. But, it is so backyardish, you will want to do it when no one is around. And when you do take the car for a drive, you should drive it like it is stolen. At least that is what I was told to do. Here is a YouTube video where the process is explained: https://www.youtube.com/watch?v=nsX6ubEVV5w

Bare in mind, the best way to fix a bad catalytic converter is to replace it, but this is a trick that helps if you are short on cash.