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#4536606 - 10/07/17 10:21 PM 98 Buick lesabre,p0420 code, o2 sensors?cat?
ziggy Offline


Registered: 08/08/05
Posts: 1487
Loc: stamford, CT
Hi again all! I just bought a used 1998 Buick lesabre, 111,340 miles,now has 111,something. Think it was sitting a while, started it, steam came out pipe,p is it was when water is in gas,or condensation? It's mostly gone. I've driven it less than 100 miles,past almost 2 weeks.ran fine. I added a bottle of humour high mileage,at 1/2 abtank,few days ago,it's almost on empty now.check engine light comes on, used a scan tool..p0420 bank 1, catalyst running efficiency. Could the humour have done something? Bad 02 sensor? Is the car shot? It looks fine from the outside,looking under car.

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#4536613 - 10/07/17 10:32 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
Chris142 Online   content


Registered: 06/05/03
Posts: 16533
Loc: Deplorable in apple valley, ca
99% of the time a 420 code is a cat. Do all the usual,plugs,wires and drive it. If it still comes back its likely the cat


Edited by Chris142 (10/07/17 10:33 PM)
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#4536618 - 10/07/17 10:36 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ziggy Offline


Registered: 08/08/05
Posts: 1487
Loc: stamford, CT
That's what I figured.I do know,the dat makes a rattling noise when it starts,but goes away after a minute or 2. Bad cat? Sensor?

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#4536631 - 10/07/17 10:47 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
Leo99 Offline


Registered: 03/30/14
Posts: 2876
Loc: NJ
Could be the cat but 02 sensors are a lot cheaper. Does your scan tool have the capability to monitor the 02 voltages while you drive?
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#4536636 - 10/07/17 10:56 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ziggy Offline


Registered: 08/08/05
Posts: 1487
Loc: stamford, CT
No it dosnt it just scans,tells you the code,and the interpretation of the code.

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#4536639 - 10/07/17 10:58 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ziggy Offline


Registered: 08/08/05
Posts: 1487
Loc: stamford, CT
I am thinking of doing the downstream o2 sensor..should I try replacing upstream as well?or just the downstream?see what happens.

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#4536655 - 10/07/17 11:18 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
71Chevyguy Offline


Registered: 07/10/11
Posts: 414
Loc: Martinsburg West Virginia
Just a word of warning if you aren't familiar with the vehicles that have a 3800 engine. If you see steam coming from the exhaust, you may be having intake gasket issues. If you see white smoke come out of the exhaust behind you when you rev it hard, then the upper intake and/or upper and lower intake gaskets are bad and have to be replaced immediately. If you do end up getting the gaskets done, make sure to get both upper and lower intake gaskets done. If you let them go bad, the engine will fill with coolant and seize while it is running. It happens to all 3800 engines and many people have no clue. If you replace everything with the best, newest aluminum frame gaskets, it won't happen again. I am afraid that may be why the car was for sale in the first place. I just bought one with 80k on it and the gaskets were 1 or 2 starts away from locking up the motor. I had them done before driving the car and very glad I did. Would have been a total loss otherwise on a perfect car. If you do the gaskets, that car will go 300k easy. If it is burning coolant, that may also be messing with your o2 sensors and cat.
_________________________
15 Chevy Sonic RS M1 Dexos1 / Delco filter
99 VW Beetle TDI M1TDT / Mann filter
98 Buick LeSabre PHM + G oil / Bosch filter
85 Skylark 2.5
71 Nova 250

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#4536664 - 10/07/17 11:30 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ejes Offline


Registered: 01/11/16
Posts: 309
Loc: Kansas
I have a '97 LeSabre and have been through this already. Steam from the exhaust could indicate LIM gasket failure and subsequent CAT issues or failure, a common problem. Cheapest thing to start with is to check for coolant loss into the oil / oil in the coolant. If evidence of both or either, replace OEM LIM gaskets with aluminum gaskets such as FEL-PRO MS98014T, or equivalent,. While you're doing this, do yourself a favor and buy the aluminum GATES 9105119 heater hose elbow connector, or equivalent, and replace it, as the plastic OEM connector will probably break when you remove it anyway. Then replace the downstream O2 sensor with DENSO 2344087, or equivalent (I replaced the upstream too, anyway, for good measure). The CAT may still be toast, but at least you won't ruin another one if you want to keep the car.

Bottom line, you probably will find it is the LIM gasket and you have to fix it or you'll keep going through O2 sensors and CATs at the very least if the engine doesn't fail first. Fix it, and that 3800 engine will last another 200-400K miles all things being equal.

Huge amount of info here if you have the time: GM 3800

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#4536673 - 10/07/17 11:37 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ejes]
71Chevyguy Offline


Registered: 07/10/11
Posts: 414
Loc: Martinsburg West Virginia
Yes! Thanks for adding the info about the coolant elbows, I forgot to mention those. Replaced them on mine as well even though the old ones didn't break or leak. Dealer forgot to tell me that mine needed a tranny when they sold it to me. Might explain why they didn't allow test drives.
_________________________
15 Chevy Sonic RS M1 Dexos1 / Delco filter
99 VW Beetle TDI M1TDT / Mann filter
98 Buick LeSabre PHM + G oil / Bosch filter
85 Skylark 2.5
71 Nova 250

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#4536676 - 10/07/17 11:40 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ziggy Offline


Registered: 08/08/05
Posts: 1487
Loc: stamford, CT
I read a little,about the gaskets.I've checked the oil,and coolant,few times already no mixing of them. The guy seemed an honest guy. It was a car for his nephew,21, but ran off for some girl in texas,so the owner,sold the car,too me.said his nephew had no interest in it,and was just sitting their in driveway,for months now .neglect started up,till I came along.I thinks that's the water mist.it's mostly gone...it's humid hete,for oictober,in ct,other cars make the same exhaust mist....like mine does. It's only in startup,most using their after parking after driving. It's either the 02 sensor,or cat.with 111,000 miles on them...I'm hoping it's just the sensors.but....should I do the downstream?or both?

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#4536683 - 10/07/17 11:48 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ejes Offline


Registered: 01/11/16
Posts: 309
Loc: Kansas
Originally Posted By: ziggy
I read a little,about the gaskets.I've checked the oil,and coolant,few times already no mixing of them. The guy seemed an honest guy. It was a car for his nephew,21, but ran off for some girl in texas,so the owner,sold the car,too me.said his nephew had no interest in it,and was just sitting their in driveway,for months now .neglect started up,till I came along.I thinks that's the water mist.it's mostly gone...it's humid hete,for oictober,in ct,other cars make the same exhaust mist....like mine does. It's only in startup,most using their after parking after driving. It's either the 02 sensor,or cat.with 111,000 miles on them...I'm hoping it's just the sensors.but....should I do the downstream?or both?




I think you're kidding yourself...it's the gaskets...but if you only do the O2 sensors, just do the downstream first because it'll eventually go bad and you'll be back where you started and you will waste the least amount of money that way. I guess if that doesn't do it, do the upstream too; still cheaper than the CAT and having a mechanic do the LIM gaskets. Then again, you might still get the code and the CAT may be toast...

If you shadetree, the LIM gaskets aren't bad if you take your time, have the tools and some patience - less than $100; youtube is your friend in this case. Don't know what it would cost for a mechanic to do it. I'm guessing 500-1000.

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#4536685 - 10/07/17 11:49 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
71Chevyguy Offline


Registered: 07/10/11
Posts: 414
Loc: Martinsburg West Virginia
The one before the cat is more important, but if you are getting the code for the cat, that would be because of the signal from the sensor after the cat. I would do both in your situation and see if the code comes back after reset. Watch your coolant level very carefully. If it is going down at all, you will have to get those gaskets done. It isn't cheap, but cheaper than another engine.
_________________________
15 Chevy Sonic RS M1 Dexos1 / Delco filter
99 VW Beetle TDI M1TDT / Mann filter
98 Buick LeSabre PHM + G oil / Bosch filter
85 Skylark 2.5
71 Nova 250

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#4536695 - 10/07/17 11:59 PM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
71Chevyguy Offline


Registered: 07/10/11
Posts: 414
Loc: Martinsburg West Virginia
I do agree with ejes, I believe, in the end, it will be the gaskets. Best of luck with it, don't let it go if it looks like it IS the gaskets. Oh and don't put stop leak in it, that is a whole other nightmare with the same results.
_________________________
15 Chevy Sonic RS M1 Dexos1 / Delco filter
99 VW Beetle TDI M1TDT / Mann filter
98 Buick LeSabre PHM + G oil / Bosch filter
85 Skylark 2.5
71 Nova 250

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#4536697 - 10/08/17 12:01 AM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ziggy Offline


Registered: 08/08/05
Posts: 1487
Loc: stamford, CT
Lol oh boy!!! I'm gonna try the sensors. This is dexcool,right? The coolant looks lime green in color...

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#4536707 - 10/08/17 12:20 AM Re: 98 Buick lesabre,p0420 code, o2 sensors?cat? [Re: ziggy]
ejes Offline


Registered: 01/11/16
Posts: 309
Loc: Kansas
Originally Posted By: ziggy
Lol oh boy!!! I'm gonna try the sensors. This is dexcool,right? The coolant looks lime green in color...


Traditional Dex is more of an orange color, but there are some universals that are yellowish and greenish now...if you have green, your guess is as good as anyone else's as to what it is in there. If it is regular, traditional green coolant, it won't be too good on the aluminum.

If the gaskets are bad, your previous owner may have replaced the coolant with cheap green coolant to hide the oil congealing in the radiator. If that's the case, you'll have to aggressively flush / clean it with degreaser or replace it altogether. Probably be a good idea to replace the thermostat too. If you drain the radiator a little, you should be able to see if there are any milky, gunky deposits in there from oil leaks into the coolant past the gaskets. If not, just poor the coolant you drained out back into the system. However, that still doesn't mean coolant isn't leaking the other way.

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