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If P0420 is present then:
Topic Started: Apr 4 2012, 08:44 PM (5,276 Views)
dayle1960
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Fastest Hampster EVER

http://geometroforum.com/topic/4097254/

Was reading through this post because my 96 is throwing this code. From the aforementioned post, it seems to me that 3 things could cause this code. 1. oil bypassing the rings and fouling the cat. 2. Head gasket blown thus leaking antifreeze into the exhaust manifold fouling the cat. 3. A bad rear O2 sensor, possibly dropped by an inattentive parts store employee.

I have tried to make sense of mwebb's advise and will try to do a "hot" compression test tomorrow. This should show me if the rings are bad or if a valve pooped out.

It's frustrating to know that a fairly new cat, 2ys old, died. I have another member looking through his FSM on this condition. I bet he will find mwebb to be correct in his assesment of the fix to my problem. If the cat died, wouldn't I be getting horrible mpgs? Shouldn't the cat be completely plugged?

My car has well over 200K on it. I have "refreshed" the engine once and rebuilt it once. It's currently getting 47 mpg on a regular basis. I wonder if the downstream O2 sensor starts to act correctly, if I will gain mpg's?

Is there a way to test the cat to tell if it is plugged?

Does anybody know the theory behind the functioning of the O2 sensor? Would beating the honeycomb out of the cat be beneficial? I just don't understand why that would help, because the O2 sensor would still sense "bad gasses", wouldn't it?

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Stiffchezze
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Sir Metrologist

The way I understand it, The REAR O2 sensors sole function in life is to turn on the CEL if the cat is not functioning correctly. It does NOT control fuel or timing at all. ^o)
Ergo no change in FE. :(

This is why I'm going to use a spark plug non-fowler to "test" (on a long term basis) the rear O2 sensor by removing it from the exhaust stream. :whistle

This is what I have understood so far. I have been wrong before, so if anyone else can back me up or shoot me down, jump on in! ^o)
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96Geoman




Fixing Bad Catalytic Converters With Inefficiency Code P0420

(How to clean your converter)

The P0420 code can pop up if you only lose 5% efficiency!
Edited by 96Geoman, Apr 4 2012, 09:44 PM.
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Bad Bent
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Facetious Educated Donkey

dayle1960
Apr 4 2012, 08:44 PM
mwebb's advise and will try to do a "hot" compression test tomorrow. It's frustrating to know that a fairly new cat, 2ys old, died.

Is there a way to test the cat to tell if it is plugged?
mwebb gave us NEW means Never Ever Worked.

Plugged Cat. Unplug the cat at the exhaust manifold, don't loose the exhaust manifold ceramic donut. Drive it and it should drive better but LOUD.

"Would beating the honeycomb out of the cat be beneficial? I just don't understand why that would help, because the O2 sensor would still sense "bad gasses", wouldn't it?" If you "gut the Cat." then the O2 sensors will read the same and the CEL will come on again.

mwebb also has a test for O2 sensors: http://geometroforum.com/topic/3208666/ and blue rhino's test: http://geometroforum.com/single/?p=558947&t=4464045
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Woodie
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Bad cat does not necessarily mean clogged, or have any effect whatsoever on how your car runs. Mine was running great, just rich, you could smell the gas at the exhaust. Still got 40 mpg. I put a new cat on it to get it through the emissions test, by the time I finished that tank of gas, the check engine light was back on. I eventually put an extender on the O2 sensor and the light went back off.
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dayle1960
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Fastest Hampster EVER

Bad Bent
Apr 5 2012, 12:56 AM
dayle1960
Apr 4 2012, 08:44 PM
mwebb's advise and will try to do a "hot" compression test tomorrow. It's frustrating to know that a fairly new cat, 2ys old, died.

Is there a way to test the cat to tell if it is plugged?
mwebb gave us NEW means Never Ever Worked.

]blue rhino's[/color][/b] test: http://geometroforum.com/single/?p=558947&t=4464045
So I already have a vacuum gauge installed on my whip. After strart up, the gauge reads 18" rock steady. So going by blue rhinos diagnosis, my cat is not clogged. Good news to me.

Now on to more enlightening. I can now theorize that there are particulates resting upon the surfaces of the cat honeycomb which emit gasses that the O2 sensor deems "bad" thus throwing the PO420. Correct????

Or I could just have two bad O2 sensors. Unlikely, but possible.

Would adding or subtracting timing be a possible solution? If the engine is running lean, then backing off the timing the leanness would flatten out and the O2 sensor would become happy again? Is that possible?

Just throwing ideas out there which are rattling around and seem to make a bit of sense to me. :cheers
Edited by dayle1960, Apr 5 2012, 05:54 PM.
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Johnny Mullet
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Fear the Mullet

You can even remove the O2 sensor, plug the hole, and set the plugged in O2 sensor on the passenger strut tower for now to see if it's the cat. I f the light stays out when the sensor is out of the exhaust stream, then follow Woodie's advice about the extender.
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ascensions
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Je conduis une petite voiture.

Stiffchezze
Apr 4 2012, 08:56 PM
The way I understand it, The REAR O2 sensors sole function in life is to turn on the CEL if the cat is not functioning correctly. It does NOT control fuel or timing at all.
IMHO, this is an urban legend perpetuated by the internet.

My own personal tests, and research on my 02 modifications disagree with this.

The front 02 is for short term fuel trim, the aft is for catalytic converter monitoring, and long term fuel trim.

The CEL is lit when a major shift occurs in between a range between the fore and aft sensor. This is how it can tell it can tell if the catalytic converter is working. However, it could just be a major change in catalyst efficency for example, such as switching from a year's worth of city driving to highway. Over a course of 150-300 miles, the ECU will gain new tables and the CEL will be distinguished. If it continues to stay lit, it means the rear is constantly at a higher rate than the fore sensor.

Now concerning fuel trim. The aft 02, over about 150-300 miles, builds fuel tables for the engine. It will assign a value from -10 to +10 in lean to rich. This value is applied to the fore sensor's readings during further driving. It does this in order to maintain emissions levels at a preset value.

For example, if the aft sensor is reading too much oxygen, the ECU believes it has too many emissions (too many oxides) and it may reduce the lean-ness of the fore sensor to maintain emissions within the "goldilocks zone" to maintain emissions. If the vehicle does not produce as many emissions, the engine can run a leaner fuel map allowing more hydrocarbons to be burnt.

This all gets very interesting because, it does mean, a healthy-functioning cat can produce better gas mileage, and that despite how you may try to meddle with the fore sensor, if you don't adapt the rear sensor, the computer will overcome tampering.
Edited by ascensions, Apr 5 2012, 07:47 PM.
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Deleted User
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Oh, Gawd, I really need to get into Volume 2, Section 6 . . . :news
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Stiffchezze
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Sir Metrologist

Was just thinking the same thing! :type :news
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dayle1960
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Fastest Hampster EVER

Ok, so now we are getting into the theory side of my quest.

If the cat was plugged, then the rear O2 sensor would be pinging a P0420 because it would not be within a certain range that is required by the front O2 sensor.

Since my cat is not plugged, then I am wondering why both O2 sensors I have put into my downstream bung have thrown code P0420.

All I can think of is maybe the upstream O2 sensor is bad which is causing the rear O2 sensor to try to find the "sweet spot" and since the downstream O2 sensor can't find it, the dpwmstream sensor throws a code.

OR

My cats' honeycombs have a contaminating coating which does not allow the catalyst to do its job and thus the rear O2 sensor is suppose to sense particulates which are not being produced due to the contamination.

OR

My engine is exhausting something that the front and rear O2 sensors do not like.


So in review, I think the possible causes are

1. Plugged cat
2. Contaminated cat
3. Bad rear O2 sensor
4. Bad ECU
5. Engine spewing bad particulates which the rear O2 sensor does not like
6. Possible bad rear and front O2 sensors

What do you guys think? I'd really like to track this gremlin down and come up with a definitive answer.
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Johnny Mullet
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Fear the Mullet

I would seriously try the O2 sensor extender. I see this all the time at work with certain aftermarket cats.
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dayle1960
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Fastest Hampster EVER

JM, you are probably correct, BUUUUUT, looking at my rear sensor, I notice that the wires coming out of the bottom of the sensor are almost touching the underside of the car. How far does this extender push the O2 sensor out of the bung?
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Deleted User
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Dayle1960,

I have read the applicable explanation of the systems operation in the FSM. I'm typing on the stupid iPhone, or I would detail my findings. I'll get you the explanation and solution options in a couple of days.

GG
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ascensions
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Je conduis une petite voiture.

I'm trying an extender on the rear, for MPG, not for CEL reasons... and it fits... but I have not ironed out all the bugs... but it does fit. (Both long and short versions.) I'd recommend the short one first.

Posted Image

This is the long one installed. You will need to gently pry down on the exhaust to begin with to screw in the long one, but the short one likely should have no problems.

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