Catalyst Efficiency Codes P0420 - P0430

These codes are indicative of Catalytic Converter failure, and before you get all involved in repairing your customers vehicle, you need to do some
federal emission warranty on 1995 and newer vehicles that cover certain emission components that may be the source of the failure, the Catalyst
being one of them. The least you can do is diagnose why the vehicle is setting the codes, and advise your customer on a course of action.

 
GM Emission Warranty States as long as:
The vehicle has been maintained and operated in accordance with the instructions for proper maintenance and use set forth in the owner manual supplied with your vehicle.
The vehicle fails an EPA-approved I/M test during the emission warranty period. The failure results, or will result, in the owner of the vehicle having to bear a penalty or other
sanctions, including the denial of the right to use the vehicle, under local, state, or federal law. GM warrants that your dealer will replace, repair, or adjust to GM specifications,
at no charge to you, any of the parts listed under the Emission Warranty Parts List, which may be necessary to conform to the applicable emission standards.

So, what are the main causes of the P0420-430 codes? Most often it is the converter that is not doing the job it should be, and the way to find out, is by looking
at the pre and post O2 sensors, the engine needs to be brought to operating temp, once there, get the rpm up around 2500 - 3000 rpm and hold it there for about
5 minutes to make sure the O2's are up to operating temp, after this run up, return the engine to idle, and view the O2 values comparing the pre and post readings,
the pre sensor should be switching from .1 to .9 volts crossing .450 volts at least 18 - 20 times in 10 seconds, that's about normal, the post sensor should remain
relatively flat lined about .3 - .5 volts, if it appears to be following the pre sensor, that's why the codes are setting, and, this would indicate a bad catalytic convertor.

Before we go on with diagnostic procedures, I would like to tell you a short story about a person who is a caregiver for my disabled son. One morning, she showed
up to be with my son, and knowing I was a retired mechanic, she mentioned that her check engine light was on (I know, I know, its now a MIL, or malfunction indicator
 light I still like check engine light to customers, they understand that) anyway,  I still have my code reader, so I sauntered out (LOL) to check it for her, the vehicle was
a 2006 Nissan Frontier Pickup with 115k miles, after pulling the code (P0420) I told her the bad news, it was most likely a bad catalyst, after some thought, I asked her
 what fuel she was using, and she told me whatever was cheap! After some more thought, I told her next time she was to fill up, go put a tank of chevron in it and drive
it til empty, then do it again, (I already cleared her code) before the first tank was used up, the check engine light came back on, it was the same code, this time I left it
alone, and told her to keep driving it and fill up again with chevron, after the second tank, and about half way through the tank, her, and her husband decided to take it
to the Nissan dealer, just before they pulled in the driveway to the dealer, the check engine light went out, they both looked at each other in disbelief, and drove up to
the entrance and asked them to look at the drivers window that had not worked for almost a year, needless to say when she showed up the next day, I got a big hug
and smack on the cheek. I was happy I did something useful for some one even after I had retired, they were certainly happy spending money for something except
the convertor. Oh yeah, I did check the codes again, the P0420 was now gone, evidently the chevron cleaned up the catalyst enough to make it start working again.
I've always ran chevron in my vehicles, since it kept injectors so clean, but not in the Monte Carlo, that I used Texaco premium since it had 9.5:1 compression.
 
To operate properly, catalytic converters need an air/fuel mixture that alternates between rich and lean. When the air/fuel mixture is rich, it reduces the amount of
oxygen in the exhaust. This allows the catalyst to break down NOX. But to burn HC and CO, the catalyst needs more oxygen so the air/fuel mixture has to go lean.
This allows the catalyst to momentarily absorb oxygen and trigger a reaction that burns up the HC and CO. The (PCM) flip-flops the air/fuel mixture when the engine
is warm by monitoring the rich/lean signal from the oxygen sensor in the exhaust. When the O2 sensor reads lean, the PCM makes the fuel mixture go rich. When the
O2 sensor sends back a rich signal, the PCM shortens the on-time of the fuel injectors and leans the fuel mixture. The O2 sensor then send back a lean signal, and
the PCM increases the on-time of the injectors to make the fuel mixture rich again. By rapidly changing the air/fuel mixture back and forth, the overall mixture averages
out and keeps emissions at a minimum. On some newer vehicles, a new type of "wideband" oxygen sensor (also called an "air/fuel ratio" sensor) is used. Instead of
producing a high or low voltage signal, the signal changes in direct proportion to the amount of oxygen in the exhaust. This provides a more precise measurement.

A three way catalytic converter with high oxygen storage capacity will indicate a low switching frequency (flatlined) of the downstream heated oxygen sensor. As oxygen
storage capacity decreases, the sensor switching frequency will increase. When the frequency of pre and post oxygen sensors approach a specified limit value, the
catalyst is tested. A converter reduces pollution through chemical reactions that require fuel (from the unburnt gas in your exhaust stream), heat (from your exhaust),
and oxygen. If these inputs aren't in balance, the cat doesn't work well. When a car's engine is misfiring, the overheating that results can destroy the catalytic converter.
When a car is running too cold from a long warm-up time, for example, let's say if the thermostat is bad, the same thing can happen, good-bye converter. Another way
a converter can die is from poisoning due to the introduction of silicone into the exhaust. When a car engine has a head gasket leak, even a small one that doesn't make
the car overheat, the silicone from the coolant can contaminate the converter and destroy it's ability to operate, possibly, it may be cleaned with chemicals and saved.
There are many different chemicals available, cataclean, is one I can think of, and there are many others, although if this is possible, two applications will be needed.