Question on P0420 & P1811 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 12-12-2014, 05:32 PM   #1
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Red face Question on P0420 & P1811

hello,

My Grand Prix 3.8L GT 2002, blew those two codes noted above.

For P0420( going on for the past month), the cat was changed under warranty, and the back O2 sensor was changed. The OBDII live, showed that the sensors were operating how they are suppose to. for Bank 1 and Bank 2.
Well a week after having that done the code came back. My Mechanic says there is nothing else to change.
WHAT WOULD CAUSE this code to come up??

Regarding P1811, that is coming up, in the enhanced codes, which where the P0420 shows also. Hard codes. BUT P1811 is only in the enhanced area, not the soft codes.

I was told the PCS needs changing, but reading it might not be the problem either. Any suggestions besides a shift kit to fix the problem temporarily. Cause if the solenoid will need changing, it will cost more in the end anyways.
Also, when it does shift off, which is not all the time. Mostly drives pretty smooth, it just shifts off a lil of jerk. You hardly feel it, but it'* there.

Is it a big job? how long does it take to replace and repair??

Any suggestions on both topics would be great, thanks.
Jo

Last edited by bradycat; 12-12-2014 at 05:35 PM.
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Old 12-12-2014, 08:42 PM   #2
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Define operating how they are suppose to be? Upstream sensors I believe, usually show a higher voltage vs the down stream sensor.

I think you only have Bank 1 sensor 1, bank 1 sensor 2. In english this means, 1 upstream sensor and one down stream sensor.

According to my research, your P0420 means that your #2 sensor is seeing the same thing as sensor #1. Or in other terms, your CAT isn't changing your exhaust. If that'* not the case, then something is wrong electrically between the O2 sensors/PCM.
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Old 12-12-2014, 09:44 PM   #3
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Well Sensor 1 - upstream. from what my mechanic has showed me, the numbers on the live OBD II, the numbers jump from .10 up to .8 and all around. Because it'* not cleaned from the cat yet. then once it goes thru the cat, the O2 sensor, shows steady at .1 to .2 or so. and it doesnt' jump around as much, like the first sensor. Upstream meaning. Voltage I am talking about also, as you indicated.

So, do you think it might be the EVAP system? Because my mechanic, looking under the hood, shows the emissions, and it ties in the EVAP system. Saying if the solenoid might stick in that tomato looking can that it'* in, above the gas tank?

Does that make sense?
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Old 12-12-2014, 09:53 PM   #4
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Are you familiar with P1811. Does a shifter kit last long, if I was to put one on?

Also how long do I wait to put one in. I have had this lil jerk problem, that doesnt happen all the time, and very small jerk, for about 5 years.
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Old 12-13-2014, 08:38 AM   #5
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a few folks have said it takes care of it forever, with the new dex 6

https://www.gmforum.com/4t65e-hd-147...ission-279684/

as far as the 420, there are both sensors involved not just the one. and it has to read in spec of a oem one so if it wasent a oem type one that could cause an issue too
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Old 12-13-2014, 11:18 AM   #6
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Hello JW, both sensors are running at the proper voltage. And the PCM is not acting up either, cause I had that problem before, the tall tail sign that you have a fault PCM is when your gauges on your dash, go all wacky, and it'* not.
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Old 12-13-2014, 06:29 PM   #7
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Default LIVE DATA what does......

hello,
On my scanner, under my live data. I am getting...
STFT B1 S2(%) NA
STFT B1 S1(%) -.07 Getting a reading here.
THEN on a separate line
STFT B1 ( %) 1.5
LTFT B1(%) 7.8

THEN UNDER CUSTOMER DATA.
it shows
STFT B1 S1 (%) 1.5
STFT B1 S2 (%) NA

PLUS MY MIL DIS(KM) SHOWS 0 (ZERO) WHEN I HAVE PUT ON 68 KMS.
Is that not wrong also??

Any help in figuring this out would be helpful, thank you
Jo



The cat and downstream O2 sensor was changed, and I am still getting code P0420.

The voltages for upstream and downstream sensor, are where they should be.

So why am I getting a NA for that one
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Old 12-13-2014, 11:13 PM   #8
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Basically, most scan tools can plot data outputs....you plot the pre O2 sensor, and it should be a rapidly changing signal from 0.1-0.8 volts...

Plotting the post o2 sensor, you will generally get a low flat signal, with an occasional High signal, but it will not be rapidly changing....

However, If the post sensor "mirrors" the pre sensor, in other words, changing the same amount, then the cat is bad.....

You say you changed the cat...the cat should be good......but one thing I have run across is, if you change your large cat, with one of those smaller universal ones, sometimes the smaller cat does not have the capacity to handle all the gasses....

Also check the harnesses to the O2 sensors that they have not grounded out against something, and that the connections are clean/tight...
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Old 12-13-2014, 11:19 PM   #9
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Because this pertains to you new cat thread, I suggest you keep that one going with what you posted above. Please try and keep all threads together to prevent confusion.
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Old 12-14-2014, 04:49 PM   #10
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Quote:
Originally Posted by bradycat View Post
Are you familiar with P1811. Does a shifter kit last long, if I was to put one on?

Also how long do I wait to put one in. I have had this lil jerk problem, that doesnt happen all the time, and very small jerk, for about 5 years.
Really best to replace the fluid/filter every 50 k miles and definitely use Dexron VI.
Over time and with infrequent Transmission fluid changes, the solenoid & valve bodies can accumulate varnish, causing build ups and cause problems.
I had P1811 and harsh shifts when hot.
Before I installed the 4T65-E Trans GO shift kit into the accumulator, I added a bottle of Sea Foam Trans Tune, which helped to to remove varnish & smoothen things out a bit after driving for 500 miles, mainly highway, to get the trans nice & hot for longer durations.
Then I pulled the trans pan, installed the shift kit, new Delco filter,/gasket & used Mibil1 Dex6, and have not had harsh shifts or the code return since.
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