What causes lean and rich out of the same bank

Dope 97GT · Mar 4, 2006

I have codes being thrown which are P0171 System Too Lean - Bank No. 1. and
P0172 System Too Rich - Bank No. 1. I know it could be a vacuum leak or a exhaust leak and i couldnt find either. Do you think i should take it to a shop and have them check it out for exhaust and vacuum leaks. Or could there be a bad sensor or something.

stng · Mar 4, 2006

That sounds like a O2 Sensor to me... The sensor probably isnt reading the exhaust accurately. If it was an exhaust leak/vacuum leak it would be throwing lean codes, not rich codes, right?

reeber · Mar 4, 2006

I have the same issue... I am replacing the O2's this week... I think it is the sensors..

Dope 97GT · Mar 4, 2006

let me know how the o2s work because my CEL is on and i cant pass inspection in NY with that on. Also my car is in storage for the winter so i cant do anything right now. And which side is bank 1. pass or drivers side

stng · Mar 4, 2006

My understanding of how O2 sensors go it like this; the car starts out in what is called closed loop when the engine is cold, which means that the car is running off a pre-set tune, and therefore the O2 sensors are not involved, but once the car gets up to operating temperature the computer goes into open loop and your O2 sensors are constantly reading the air/fuel mixture of the exhaust and adjusting the amount of fuel, timing ect. to result in optimal performance and/or least amount of hydrocarbons that the engine is producing. Most of the time when a O2 sensor fails, it completely stops working and then the cars computer goes back into closed loop, but sometimes (This happened to me) the O2 sensor does not stop working altogether and instead sends incorrect data back to the computer and then the car does not run right = too lean/too rich fuel mixtures. Assuming that that is the problem you can replace the O2 sensor, disconnect the battery for awhile to reset the computer and that should fix your problem and allow you to pass emissions. Lastly I dont know what side is bank one but if I had to guess I would say its the driverside, it is on Mopar engines...

jstreet0204 · Mar 4, 2006

stng said:
My understanding of how O2 sensors go it like this; the car starts out in what is called closed loop when the engine is cold, which means that the car is running off a pre-set tune, and therefore the O2 sensors are not involved, but once the car gets up to operating temperature the computer goes into open loop and your O2 sensors are constantly reading the air/fuel mixture of the exhaust and adjusting the amount of fuel, timing ect. to result in optimal performance and/or least amount of hydrocarbons that the engine is producing. Most of the time when a O2 sensor fails, it completely stops working and then the cars computer goes back into closed loop, but sometimes (This happened to me) the O2 sensor does not stop working altogether and instead sends incorrect data back to the computer and then the car does not run right = too lean/too rich fuel mixtures. Assuming that that is the problem you can replace the O2 sensor, disconnect the battery for awhile to reset the computer and that should fix your problem and allow you to pass emissions. Lastly I dont know what side is bank one but if I had to guess I would say its the driverside, it is on Mopar engines...

Close, but you have your loop states reversed. Open loop means it is runniing on predefined variables. Closed loop it is adjusting based on o2 readings.

If an o2 completely fails, it will go into limp mode, which is different than open loop.

Also, bank one is passenger side.

To the originaly poster, try swapping the front o2's. If the code follows the o2, you know that is your problem. If the code doesn't follow it, you've saved $50 bucks, and you know you have to look elsewhere.

Dope 97GT · Mar 4, 2006

yeah that sounds about right...Ive been havin problems with this CEL from around june. I think it was making my car run bad too but ill try the O2 sensor. First i thought it was the rears because of the O/R h pipe so i bought some MILs and they didnt work. Then i tried putting on the stock h pipe with cats but that didnt do anything either so i bought an O2 sensor and replaced the drivers side front one and it didnt do anything so i was stumped... but now i know bank 1 is passanger side so i guess ill just put the new 02 in that side and reset the comp and hopefully ill be on my way to a nys inspection which is

TGJ · Mar 4, 2006

Before you go and replace your 02's sensor's, have you:

Checked and cleaned you MAF sensor?

Checked and cleaned your EGR?

I have had this happen to me with my TBird, the problem turned out to be a faulty MAF sensor. I bring up the EGR as I had been throwing P0172 and P0175 and did everything but check the EGR to resolve it. I cannot confirm that cleaning the EGR has resolved my issue as the car is sitting for the winter.

Dope 97GT · Mar 4, 2006

well i havent dont that but i already bought 1 new o2 sensor but ill try the EGR and MAF too do u have any idea how to clean them.

Mike97gt · Mar 4, 2006

TGJ said:
Before you go and replace your 02's sensor's, have you:

Checked and cleaned you MAF sensor?

Checked and cleaned your EGR?

I have had this happen to me with my TBird, the problem turned out to be a faulty MAF sensor. I bring up the EGR as I had been throwing P0172 and P0175 and did everything but check the EGR to resolve it. I cannot confirm that cleaning the EGR has resolved my issue as the car is sitting for the winter.

+1

many times the MAF sensor can cause these codes or even the egr system.

remember the 02 sensor is a secondary sensor as far as fuel ratio is concerned. the PCM puts much more weight into maf and load percentage than into 02 readings.

I would start looking at the MAF first

CanadaStang · Mar 4, 2006

Yip, always check the cheap/easy to do things first. Here's a great write-up on cleaning the MAF:
http://www.stangnet.com/Tech-Articles/Cleaning-The-MAF-Sensor.html

Dope 97GT · Mar 4, 2006

Now about the EGR valve what could be the problem with that. Do you think it needs to be replaced or the EGR valve sensor? What should i do with that?

Mike97gt · Mar 4, 2006

Dope 97GT said:
Now about the EGR valve what could be the problem with that. Do you think it needs to be replaced or the EGR valve sensor? What should i do with that?

well most of the time if the egr was causing a code like this you would also have misfires to.

the maf is the most likely

Dope 97GT · Mar 4, 2006

when i orginally checked the codes when the CEL came on i had a misfire in cylinder 1. But i cleared the codes and tried a couple things and the misfire code didnt come back. Do u think that could mean anything with the EGR?

Mike97gt · Mar 4, 2006

Dope 97GT said:
when i orginally checked the codes when the CEL came on i had a misfire in cylinder 1. But i cleared the codes and tried a couple things and the misfire code didnt come back. Do u think that could mean anything with the EGR?

it could. but i would also check the coil and plug on cyl 1

a misfire can set a lean code for that bank, because the 02 sensor will pick up the unburned oxygen. it will not sense the unburned fuel.

if your car is misfiring on cyl #1 then you can pick up a PO301 misfire cyl number 1, a po171 for bank 1 being lean due to the unburned oxygen. and to top it off a po172 and 175 ( bank 2) can also be set later after the car has been driven for awile, when the unburned fuel from the misfire in cyl #1 starts to drain into the crankcase and mix with the oil. these fuel vapors being sucked thru the pcv system will cause the mixture to go rich.

how long have you been driving with a misfire.

to see if this happend pull your dipstick out and smell the oil. is it thin and smell like gas?

Dope 97GT · Mar 4, 2006

The misfire code happened a while ago. I thought it was because of my wires and plugs I am about to change them. But i have not been driving on a misfire the code only showed up once and then it has never came back just the lean and rich codes. But im gonna check the MAF and clean that and change around the bank 1 and 2 O2 sensors and see if the code follows the O2 sensor. and i have a new O2 sensor if it is that. All these trips to the mechanic i should really just buy the SCT2 and read them myself and get some better times with a tune but im about to sell the GT and get a 01 cobra vert in the spring .

Mike97gt · Mar 4, 2006

yeah check the maf. for that other stuff to happen you would have to have a constant misfire.

sounds like the misfire you had went away?

What causes lean and rich out of the same bank

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