PO420
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Joined: Feb 2008
Posts: 12
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PO420
I got this code from my 2002 LS. The cat was changed back in August of 2007 with an aftermarket one. I've read technical bulletins about how aftermarket cats can trip the "check engine" light to go on. Anyway, the check engine light has gone on twice in the last week. I put a scanner on it and it gives me the 420 code. I've cleared it everytime and it goes back on in about a day or two. What other options amI missing?Do I need to get the original GM part? Thanks.
Monte Of The Month - April 2008
Joined: Nov 2007
Posts: 1,406
From: Waco, Tx
RE: PO420
I "think" there is a way to bypass it, but I can't tell you what/how. Do a search on here and you should find it. But the GM cat would/should fix it. Lot's of people that have put on aftermarket cats have had your problem.
Joined: Jan 2008
Posts: 59
From: Texas
RE: PO420
If I am not mistaken part of that code also indicates that it could be the downstream O2 sensor. I have an aftermarket cat on my firebird and buick with no problems. You may want to check with the person who installed the cat it is probably still covered by a federal emissions warranty which does also apply to aftermarket cats.
Monte Of The Month -- August 2007
Joined: Sep 2006
Posts: 5,231
RE: PO420
If you had under 100,000 miles when you changed the cat', GM will pay to replace your cat' with a replacement cat', but not a high-flow cat'. If you have over that in miles, the go with the O2 emulator from ZZPerformance or any of a number of sources.. I have the same problem with my '04, but it's not that simple for me. I'm going to try an O2 bung extension to pull it out of the flow some. Hemi's do this.
RE: PO420
Hi Mike,
Below is some additional information
[:-]
Information about DTC P0`420 "Catalyst System Efficiency Below Threshold"
More info on the error code...
http://www.troublecodes.net/articles/catfailure/
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.
[align=center]"Three-way catalytic converter (TWC) efficiency is measured by how well it can store oxygen. [/align][align=center]http://www.troublecodes.net/articles/catfailure/
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.
"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. [/align][align=center]
[font="times new roman"]"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
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