Note: Oxygen sensor 1 is often informally called the "primary" sensor, or "upstream" sensor. It's installed after the exhaust manifold, but before the catalytic converter. Oxygen sensor 2 is informally the "secondary", or "downstream" sensor. It's installed just after the catalytic converter. If you hear people mention those terms, that's what they mean.
With the extreme sensitivity of current North American emissions controls, and the extremely low emissions limits that have been imposed, certain errors now crop up that didn't happen before. One of those is the P0420 error, which is supposed to mean that the catalytic converter is not working as well as it should. Unfortunately, it just means the downstream O2 sensor thinks too much oxygen is sneaking past the cat and tattles to the ECM.
A catalytic converter is an oxygen storage device. The only way it can function is if it can take up and release oxygen in the quantities required to convert engine emission gases to water and carbon dioxide. OBD-II specifications require that the catalytic converter be regularly tested by the ECM. Test failures tell the computer that the cat has lost some of its oxygen storage capability, which is what's meant by "below threshold".
Your car's computer is allowed to adjust the fuel/air mixture within a very narrow range in order to help keep the P0420 error from happening. If the needed adjustment exceeds that range, the error code will be set.
The following quote is excerpted fom an unknown GM service guide. I collected it off a Usenet post. It's generally applicable to any ODB-II car.
"Three-way catalytic converter (TWC) efficiency is measured by how well it can store oxygen.
"The ECM monitors converter efficiency by comparing the voltage values of the heated oxygen sensor 1 (HO2S-1) and heated oxygen sensor 2 (HO2S-2). Under normal operating conditions, the HO2S-1 should vary between 10 mV and 1065 mV and the HO2S-2 should remain relatively steady between 500 mV and 800 mV. This steady reading of the HO2S-2 indicates a correctly functioning catalytic converter.
"When all parameters have been met, the ECM will run a 5 second DTC P0420 diagnostic at idle. The ECM will
command rich and monitor the time it takes the HO2S-2 to go rich. It will then command lean and monitor the time it takes the HO2S-2 to go lean. The longer it takes the HO2S-2 to change rich/lean means the converter is storing oxygen and is functioning properly. If the five second test fails, the ECM may take several tests during several ignition cycles to set the DTC. DTC P0420 sets when the ECM has determined that the catalytic converter is no longer efficient."
The typical professional response to this error is ultimately to replace the converter. The catalyic converter is, in the US, covered under an eight-year, 80,000 mile federally-mandated warranty (the cost of which is figured into into the price of the car), so there's not much point for dealers to do much other than replace it. If you're outside the warranty limits though, you've got a bit of a problem, since OEM cats cost over $1,000, and aftermarket ones often aren't as durable. And you can't pass an emissions test with an error code stored.
If your state or province has no emissions test, and you don't mind staring at a yellow light all the time, you could just keep driving it without causing damage to anything. So long as you were certain that the only error stored was the P0420 one, that is...
Another possible cause, based on my readings of posts to the newsgroups. I do not know how accurate it is, but there seems to be a pattern in peoples' reportage of the problem, which is why I decided to add it to this page.
It appears from various posts in several groups I monitor that this error code is most common on vehicles that spend much of their time at low speeds, on short trips, and doing city driving. It's possible that the catalytic converter in such cases rarely gets hot enough to help burn off contaminants, and those contaminants end up preventing the chemicals in the exhaust from reaching the catalyst element. A long, hard, highway drive may cook off enough of the contaminants that the cat may start working well enough again to turn off the light.
Drive at a time when you can get up to the highest speed limit thats legal in your area, and stay there for an hour or more. Add load to the engine by (all at the same time) turning on the A/C, the heater fan full-blast, the headlights, and open all the windows to increase drag. Sounds silly doesn't it? But doing all these things will make the engine work harder, which makes it use more fuel and thus better heat up the catalytic converter, which may help it cook off the crud that led to the error.
Note that it may take several engine start/engine stop drive cycles afterwards before the light will turn off.
If the above doesn't work, even if you try it more than once, then the cat probably really has permanently lost peak efficiency, and you may have no choice but to live with the yellow light, or get the cat replaced.
NOTE : I LIFTED THIS PAGE FROM THE INTERNET WHICH SAVED ME TYPING THIS OUT BECAUSE ITS PRETTY MUCH WHAT I WANTED TO SAY