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Information about DTC P0420
"Catalyst System Efficiency Below Threshold"
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Apparently, on certain Hondas this can be due to a
cracked
exhaust manifold, but this is fairly rare...
More
info on the error code...
http://www.troublecodes.net/articles/catfailure/
And
still more...
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.