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Old 04-02-2012, 07:25 PM   #1
ryszardsh
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Default P0420 code cured

I have a 98 and have been chasing a very intemittent P0420 cat efficiency code for months. Background is new O2 sensors last fall, along with new valve cover gasket, new air pump hose (broken while installing) new cam sensor.

Since December, every once in a while I would get a cat efficiency code and the CEL comes on. Car still runs well and gets good mileage. Never got any data into the data storage in the computer when the code would show up. Ran for a while with an OBD monitor plugged in and recording and even tho code/CEL came on during test, no obvious anything in data as to why. If I drove around in 3rd gear (car is an automatic) no code would pop and CEL did not illuminate. Really beginning to think some $ for a new cat would be needed.

Found a thread here that suggested the engine temp sensor might be the culprit. Have no clue why that was suggested but it has cured my bogus P0420 cat efficiency code. Several days running now, no problems/no codes/no CEL. Anyone got any clue why/how an engine temp sensor (the 4 wire unit in the block itself) could/would cause this kind of a problem? I pulled the old sensor apart after removal, did not see anything obviously amiss. Happy but still cannot figure this out.

RAS

PS, new sensor has not changed where car temp gauge reads and as far as I can tell has not changed the reported engine temps on the system, pretty consistent 98-99C on the water temp b/4 and after change. I have not run the temp test for long since the change and intend to run a 30 minute record temps with the OBD unit plugged in and recording, not that I expect any surprises.
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Old 04-02-2012, 11:39 PM   #2
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Because if the temp the computer is reading is wrong it will throw the A/F ratio off and make everything go a little nuts.
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Old 04-20-2012, 05:16 PM   #3
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Default P0420 code cured - NOT!!!!!

Well the dang #$$%^&$*&#@ P0420 code has come back!

Trying to find out why I have monitored my oxygen sensors for quite a while on a drive last night. The rear sensor, as it should, cycles back and forth regularly between .4 v and .6 v (not dead sure on the units, but that is pretty much the correct range I think) most of the time, with a fairly regular cycle - except that every once in a while - 3 or 4 minutes with no apparent regularity the reading drops instantly to -0- (or almost zero - I am watching a charted read out, not a numerical one) and back up again immediately. A reading of 0 or very close tells the engine computer the engine is running far too lean. In watching the short term fuel trim (altho I cannot watch it at the same time as the sensor output readings) I see a strong drop into the high negative numbers, with about the same frequency as the sensor voltage readings. I see some correlation between the events and throttle position - an accelarting throttle position never causes the drop, a closed/decelerating position is always where it happens.

I have no doubt that this bouncing reading can cause the CEL/P0420 issue - an iefficient cat will show high or low O2 as a result of flow issues. I do not think I have a bad cat because the event is so transitory - a collapsed or damaged cat would not change output that quickly - I think I have either a bad O2 sensor output or something else - and that is my ?

I am trying to figure out if this is a chicken or egg issue - is the sensor malfunctioning somehow ( an intermittent short on the signal line would cause what I see, for example) or is it simply reading what the computer is doing with the short term fuel trim and reporting it?

Anyone know if the short term fuel trim is merely an extrapolation from the O2 sensor or is it really reporting what the computer is doing to the fuel trim or is it reporting the fuel delivery, not reporting what the computer is trying to do to fuel trim?

If it is an extrapolation from the O2, then I think I swap the O2 and see if that resolves. If it is merely reporting actual fuel delivery then maybe I have a fuel pump/pressure/regulator issue (or a bad injector altho that should report as a problem with a single cylinder also?) If neither do I have a computer issue or something I have not thought through adequately where the system computer is driving the short term fuel trim very low every few minutes for an instant for some reason?

I have closely checked the entire intake system with a can of brake cleaner - no leaks with that test, so I do not think I have an intake leak causing this. The car has absolutely no other apparent issues - runs fine, idles well, except my mileage has dropped a bit over the last few months with this problem - as I would expect if the computer thinks the engine is running too lean and is compensating very slightly richer to balance. The only other clues are that running the engine harder does not seem to ever trip this - if I leave the car in "3", not "D" (it is an automatic) then the CEL/code never appears. Also, it most commonly appears (and the readings bear this out) when the throttle is closed or almost closed, as when decelerating from speed before braking.

Chasing intermittent faults is so darn much fun. All thoughts/ideas welcome!

RAS
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Old 04-20-2012, 05:24 PM   #4
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98 was the year of the defective cats. There was a 10yr warranty on them where BMW should have recorded it if your cat had been replaced under that TSB. The dealer was able to tell me about mine when I gave them the VIN.
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Old 04-20-2012, 07:00 PM   #5
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I was getting similar issues, and it ended up being the maf. Car ran fine. The low end maf readings were off just enough to not throw a MAF code, but screwed up the O2 sensor readings and tossed a few other codes.


However..... I would check the rear O2 sensor wiring also. Mine was hacked up from running over something. Forward of the connector.....
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Old 04-20-2012, 07:57 PM   #6
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Never tried this, but seems like it'd be worth a try:
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Old 04-20-2012, 09:05 PM   #7
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in order:

Dave, I have owned the car from new, it was not covered under the cat recall - I checked a bunch of years ago and my VIN was not affected. Gotta admit now I am wondering but too late anyway.

Truckster, I will check the wiring, did not think about that. Both front and rear sensor were replaced last fall at a 125000 mile things are wearing out fest....got to think through the MAF.

Jess, Jeez Louise, I may just spray a can of seafoam down it and see what happens, not sure I want to run laquer thinner through it, wonder what the octane is of that stuff

Odd thing is that the pre-cat sensor seems to read normally up/down/up/down almost a sine wave except when the throttle is closed down from a cruise setting, when it drops to near zero volt reading also. That sort of makes me doubt I have a bad rear sensor. Maybe this is a MAF problem. The sensors read relative rich/lean state of the exhaust as it enters and leaves CAT, right? If exhaust is too lean going in (reading zero O2), then has to come out too lean also.

Really wish I had a logic map for the computer and knew how it views that state - this even has me thinking that maybe my secondary air pump comes on when it should not, pumping excess O2 into the exhaust sometimes and effectively leaning out the exhaust stream.
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Old 04-20-2012, 10:10 PM   #8
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It makes sense with closed throttle decel to read 0. The fuel gets shut off completely.

What do the fuel trims look like?

OK, reread everything. If the car is thinking it's lean, it's the MAF. Same issue I had. See if you can find a used one from a member.
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Old 04-29-2012, 11:45 PM   #9
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Default Really cured this time

A big thanks to Familytruckster and a bit of egg on my face...

The rest of the story: Truckster suggested it may be a problem with the MAF, so I take a look-see. At first I do not see anything odd, no weird readings from monitoring. Then I dig into the area around the MAF, thinking maybe I have a loose wire or something - and I see an odd orange area at the top passenger side rear of the airbox...Thats odd, that looks like the side of the air filter. I unlatch the cover and discover the latch under the air horn that cannot be seen is not latched, the other 2 latches, while on top of the airbox are not properly latched and the entire top of the airbox is loose, letting the air filter media pull up out of the air box. So I test this, I start and run the engine and when the engine revs it shifts ever so slightly in the mount and the top of the box seals down - when the throttle is closed quickly the engine shifts back on its mounts ever so slightly and the top of the air box opens - letting the engine pull in hot, unfiletred air from the back of the radiator! I correct the problem, put the filter back as it belongs and voila, no more bad cat code! A week plus driving, no code!

I think I now have found the problem - in January, ahead of a series of road trips when I did not have time to do it myself, I took the car to a local shop I use some for an oil and filter change. Their system has them check the air filter, vacumn the car etc - and I have no doubt that the young man that pulled the filter (even tho I told them I had changed it 6 weeks before) to check it did not get the airbox latched correctly and did not get the filter inserted correctly in the airbox. The code problem started about the same time.

My thinking is the engine shifts in its mounts ever so slightly on deccel and when it does it opended the airbox, as I saw, and allowed the engine to swallow much hotter (and therefore thinner) air from the backside of the radiator. The engine is reading the air temp probe in the airbox but getting much hotter thinner air every once in a while. Most of the time the box was sealed well enough to not pull hot air, only as the engine shifted in its mounts toward the driver side on deccel did the filter box open up enough to allow in the hot dirty air. I never thought to check the intake air temp, and probably would not have seen it even if I had b/c the opening was so transient.

So big thanks to Familytruckster for getting me to look at the MAF/intake side of the system and some red face on my part for letting someone touch my engine and causing me a weird problem!

RAS
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Old 04-30-2012, 03:41 PM   #10
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Crazy story, but it proves that the engine management is a very delicate system.

When owners try to 'improve' things with CAI and other 'mods' it often can do more harm than good.

Glad you finally sorted it out! Definitely a strange one!
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Old 05-01-2012, 12:25 PM   #11
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Wow. Glad it's fixed!

Sent from my ADR6300 using Tapatalk 2
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Old 05-14-2012, 07:43 PM   #12
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Just to add a link, this page:

http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm

has, about 1/2 way down the page, a very nice description of the P0420 code, why/how it generates and an explanation of possible causes. Makes clear that the code generates from a comparison of the rate at which O2 is converted by the cat, if I read this correctly, from comparison of rate of change of O2 content pre and post Cat. Frankly interesting in and of itself.

RAS
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Old 05-04-2018, 02:27 PM   #13
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Sorry to resurrect this old thread. But we have literally the same problem ... P0420, and the rear clasp on the side of the airbox was bent and not locking. I cannot believe this fixes a P0420. Curious if anyone else has run into this. In the meantime we'll reset our code and see if it goes away. Our airbox is now completely latched down.

Filippo
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Old 05-09-2018, 02:18 AM   #14
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So we went ahead and fixed the clasp. Still getting P0420. We put new Bosch upstream and downstream oxygen sensors in the car in the last 30k miles. Replaced the MAF as well. Continues to throw P0420. Most things are new on the car, and it runs like a swiss watch. Thoughts?

Filippo
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