P0420 and bad oxygen sensors

I take no position about replacing or not replacing the bleeping
bleeped up cat but this had been a very interesting and informative
discussion....

 

Removing the cat won't serve a purpose because without the cat, you will
have a huge exhaust leak. Even if the cat is not working, you would be
better off leaving it in place until you have saved up enough money to
replace it. Cheap aftermarket cats start in price from around $75 Of
course, you get what you pay for, and those cheap parts will probably only
last long enough to pass an emissions test.

 

Why? It has no effect on the running of the engine - no effect on
power or mileage. (mabee a BIT of exhaust restriction)

 

And it needs to be replaced, why? It will not effect the running of
the car - and removing it cannot be done legally, nor can you fool the
control system into thinking it is there if it is removed.

Find the money, drive it as is, or park the car.

 

With OBD2 the sensors are virtually self checking - There ARE
conditions that can slip by, but they are rather rare. On pre-obd2
cars a lazy sensor would not be caught - but the computer knows what a
GOOD sensor should do, and if the signal does not behave the way it is
expected to, it gets flagged (too few counts, or too narrow a range).

Almost unheard of in the real world. There is no documented failure
mode of the sensors that could indicate a non-operative cat when in
fact it was working. Now, this does NOT mean that the cat might not be
functioning due to outside forces - but in that case you WOULD get
other codes. The measure of a tech is if he knows which codes are
primary, and which are secondary. A good tech can almost "smell" which
is the cause, and which is the effect. Testing pinpoint charts cannot
do that - it comes from experience and understanding.

If you follow the troubleshooting chart in the manual you will likely
EVENTUALLY find the problem - but you will go down a lot of dead ends
first - and may replace a few parts that are not required.

A GOOD tech will start at the other end, and rule out whole blocks of
tests at once, without having to replace good parts - and he'll reach
retirement age with a full head of hair.

 

All of this discussion came about from an article posted that said to not be
too quick to replace a CAT due to a P0420 report. Among other causes, the
author makes clear that the sensors get lazy.

<quote>
Lazy oxygen sensors can distort the data and confuse the PCM.

</quote

That's precisely what I have been saying.

Unless one has done something to contaminate a CAT, a lazy sensor is going
to report a false condition. In this instance, the after-CAT sensor will
report that the CAT is not doing it's job. I'd be looking at the sensor
first before I'd be ordering a new CAT.

 

A lazy O2 sensor will not set P0420. P0420 is set when sensor #2 sees a
"lively" signal, indicating that the cat is not working. If the cat is
working, the signal from sensor #2 would seem kind of lazy, and the "lazy"
signal would indicate a good cat.

 

Different manufactures have different strategies in the pcm to test the
cat. If the strategy is to drive the exhaust lean/rich or rich/lean and
measure the time delay between the oxygen sensors, then a lazy front
sensor could set a p0420/p0430.

 

Actually a lazy after cat sensor would be more likely to pass a bad
cat than to condemn a good one.
The front sensor clocks, and the rear one doesn't (because it's lazy)
gives you a false pass, not a false fail.

If the front sensor gets lazy and doesn't clock, OBD2 sets a code. If
it narrows it's response, OBD2 sets a code. If it fails biased rich,
the vehicle runs too lean and GENERALLY sets a misfire code. It would
not set a catalyst code. If it bises lean, the engine runs rich so the
oxidation catalyst MIGHT lose efficiency, in which case it COULD set a
code.

So yes, it is POSSIBLE that a bad sensor could cause a catalyst
failure code, but EXTREMELY unlikely - and it would be the PRE cat
sensor, not the post. A postcat sensor failure will almost inevitably
cause a sensor code, not a cat code. There is virtually no sensor
failure mode that would cause a post-cat sensor to clock if the cat is
functioning.

That said, a perfectly good cat can fail to function under certain
conditions - like an exhaust leak ahead of the converter - but that
will GENERALLY throw a code for rich limit exceded or something
similar. (sensor says lean, engine attempts to richen to correct, and
goes beyond a predertermined limit attempting to correct a problem
that REALLY does not exist)

So yes, while it is POSSIBLE a bad sensor could

 

A lazy front sensor, yes. Rear sensor, no. And OBD2 standard virtually
dictates the front sensor will code before the cat under this
condition.

The only likely exception would be a vehicle with a linear O2 sensor
(LAF) because it doesn't clock the same. It can maintain a constant
A/F ratio instead of "averaging" - but it would require a different
cat setup. The traditional 3 way cat REQUIRES rich and lean transients
in order to function. No lean transitions, and the CO/HC oxidation bed
does not work - no rich transitions and the NOX reduction bed does not
function.

I'm not sure how constant ratio systems handle the catalyst, but I
know pre-OBD2 Honda Civic VX vehicles have a BEAR of a time passing
E-test in Ontario. They run terribly lean, and generally fail (HC?)
dismally.

 

True, but in that scenario, the codes were not caused by the post-cat
sensor.