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Old 07-11-2010, 01:28 PM   #1
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P0431 issue, replaced front downstream, still issue

So Ive been getting this P0431 issue for the past 3-4days and been trying to nail it down. Hooked up my OBDII software with realtime and it shows that Sensor2 (front back downstream) is running flatline at idle. Attached is a chart to show what I mean with live view.



Now it shows flatline like that when its idling. Once you get on the gas and start moving everything (all 4 O2s switch beautifully up and down like a nice sine wave). Only at idle does this O2 just flatline at 0.8v area. The other 3 switch a bit erratically but they still switch which is the key.

Now we replaced the front downstream with a new one as its the cheapest fix first. Did another live scan, and what you know, nothing changed, the above chart is identical when sitting at idle.

So to me seems like maybe the front mini cat is bad. The car as 160k miles on it so it could very well be bad and I have no chance of warranty since emmissions is 8/80k. But these damn wranglers the mini cats are part of the Y pipe and the main cat is all one piece as well. Thats about $500+ dealer part, aftermarket is about $375 area. Still expensive for a cat replacement. May think about maybe cutting the mini cats off, get some replacement ones from say jegs or summitracing and maybe weld some on in place, not sure about it since its going to be a bit tricky getting it all lined up and crap.

What you all think? Think it can be the cat? The car runs basically fine, idles fine, dont see any major fuel issue where its misfiring or stuck injector or anything.

Dixit

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Old 07-11-2010, 02:01 PM   #2
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Make sure you dont have an exhaust leak

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Old 07-11-2010, 02:54 PM   #3
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Check for an exhaust leak - O2 gets on the sensor - remember the exhaust pulsates between pressure and vacuum - sucks in cold O2.

Check cat - run at 2500 RPM for 2 minutes - use remote infrared temp gauge - should be much hotter going out the cat than going in. Should be able to see a difference between the 2 pre-cats if 1 is defective - and across from input to out of the main cat there should be at least a 250 degree difference.

What software are you using?


Check the O2's heater circuit - wire for it may be broken somewhere.
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Old 07-11-2010, 04:22 PM   #4
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Definitely no exhaust leak anywhere that we can see. That I can be sure of after double checking it.

As far as exhaust pulsates, I agree but when comparing this front bank downstream with the rear bank downstream, they are night and day. Front bank downstream is like that chart, flatlined at 0.8v. The rear bank downstream moves up and down between 0.9 and 0.1 basically like a normal O2 should switch. Software Im using is the old Harrison OBDII software. I got it awhile ago and havent purchased a new one since it works. It provides all type of details and live sensor readings like that chart which reads like 10x per second.

As far as heater circuit, dont believe its that as it would throw a code for that if it didnt heat properly, it has codes for low/high voltage circuit for heater. When you leave it unplugged it throws a code like low voltage circuit malfunction. Remember I just put a new front bank downstream and that didnt change the flatline line voltage im getting at idle for that front bank downstream. Im fairly positive that the P0431 is because its not seeing proper switching from that sensor, a flat line is basically no switching.

Again once I get on the road or even put the slightest bit of load,the sensor acts 100% normal, just doesnt seem to switch like it should like its partner one (rear bank downstream) at idle.

I dont have a infrared temp gauge, but its something ive wanted to get so this may be an opportunity to snag one as an excuse that I need one.

Dixit
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Old 07-11-2010, 10:58 PM   #5
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Harbor Freight has the infrared temp gauges on sale. Mine has lasted 3 or 4 years - good deal for the price. And, they are so handy!

Dark night or dark garage - have someone spray light oil or carb cleaner down the throat (running and warm) while you go under with a strong pencil beam flashlight. Look for smoke - exhaust leak. A small leak could be enough to affect the sensor but not be audible - hard to find other ways. Manifold at the head, welds on the precats etc - anything upstream of the sensor.

Flat line at nearly 0 voltage - good way to remember it is the Oxygen "Absorbs" the sensor's output voltage. The more O2, the lower the output voltage. (The O2 sensor acts like a little battery generating voltage once it's warmed up.)
Pinned at near 0 indicates to me it's seeing too much O2. The one after the cat acts more normal because the cat's burned some more of the O2.

Try blowing some propane into the intake - letting the engine burn more O2 because of the extra fuel. The flatline will come up. Ether - starting fluid or gasoline will also do it - BE CAREFUL!

A misfire puts too much O2 in the exhaust, or too lean, or exhaust leak. You said it's running nice, which eliminates misfire and lean.

Let us know.
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Old 07-11-2010, 11:48 PM   #6
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Yeap thats exactly where I was going to get one from (the infrared temp gauge).

We will try the carp cleaner down the throat and see what happens there and see if we get any smoke, i got an infrared/ultraviolet light as well which will help light up smoke.

Well you can see from my graph that the voltage is flat at 0.8voltage, not really flatlined in that its zero, I meant flatline that it just sits near the 0.8v area and doesnt switch up and down like it should. So from what you are saying then 0.8v would mean its not seeing much O2 at all. What dont make sense is why is this only troubling at flipping idle, once driving or engine is loaded, that damn sensor via live chart shows a perfectly healthy switching O2, nice up, down, up down sine wave. Just at idle it sits flat at the 0.8v area. We know it cant be the O2, since we replaced it. Dont believe its wiring or anything since it works fine when driving, it just doesnt like it at idle.

We've been banging our heads on this issue for hours upon hours. Removed injectors, tested them, removed sensors, tested them, bunch of crap trying to eliminate things. One thing that just boggles us is that everyime time we touch this jeep it seems to get pissed off and decides to throw another random code at us when we havent even touched that part such as 1hr ago we got Cyl6 misfire, we know that damn plug works fine, we even pulled the coil reseated it, same thing, pulled the coil/plug and tested it out and it sparks fine. This is not a full code yet, showing in pending status.

We're now starting to feel the PCM is acting really flakey, it just randomly is acting wierd like this. So we going to take your advice from another thread of leaving the battery disconnected tonight for over 10hrs and see what happens after it completely kills the PCM flash and see what happens from there.

Otherwise next options (more expensive) is to start replacing the damn coil, replace the flipping PCM, then see what happens.

Dixit
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Old 07-12-2010, 04:22 AM   #7
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OOps - Sorry, I thought it was flat at near 0. You are right, .8 is close to the top of the range - indicating low O2. Says Rich?

Carb cleaner, propane etc will only make it richer, probably very little effect.

That also precludes an exhaust leak on the flat bank. But if the exhaust leak was on the other bank, that O2 sensor tells the ECM the system is lean, then the ECM would richen up the whole system since it cannot just richen one bank only. The non flat side would toggle normally (reading the false oxygen,) but the other side would see rich - as it does.

The non-flat side would be acting normal, toggling normally trying to compensate for the exhaust leak. But since the entire system richened up the flat bank would "see" reality and indicate too rich.

How 'bout unplugging an injector on the bank that's showing flat? That should put extra O2 into that bank and cause the flat bank to start toggling, or at least go high? That would prove the sensor is doing it's job correctly.
The engine will probably run very rich when you unplug, and a lean code may set. Funny how sometimes you have to think backwards.

He He - in other words, look where it "seems" OK.

I never tried an infrared light to look for smoke. Seems like the smoke would have to convert the light to visible light to see it. But with the yellow goggles it may be interesting. I'll have to try it.

I don't think the 10 hour deep cleaning will help in this case, but it's certainly worth a try.
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Old 07-12-2010, 10:06 AM   #8
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We going to have to try this method you described of tricking the ECU temporarily by unplugging the injectors. I know while we were testing it to ensure no injector was stuck open or closed, we pulled each injector plug one by one and listed to the engine and we determined that it was definetely working as normal, but we didnt look at the live O2 readings which is what we will have to do now to make sure its not richening up the system, and like you said see if the O2 sensor does start switching as normal.

Damn the 10hr cleaning wont help here? hahhaa was hoping something would cure this flipping problem. Guess we just going to keep drilling away at this random issue thats pissing me off a bit.

Dixit

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