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Check engine light on intermitenly,scanned and code was for lean left bank.replaced both O2 sensors. light then went off but came back on after a few miles and days. scanned again and it says the o2 s

I've checked all connections and found nothing loose. The light did go off once but came back on after a few miles.

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  • cllash79 Dec 04, 2012

    Codes from scan. CO265 PO171 Bank 2, PO174 bank 1

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  • Expert
  • 318 Answers

O2sensors reading high or low depending fuel goin in. fuel filter is a good place to start for low or lean fuel.

Posted on Dec 03, 2012

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6ya6ya
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Posted on Jan 02, 2017

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SOURCE: Check engine light was on. OBD II scan said both

The pre cat sensors running lean does not mean they need to be replaced. They are are detecting a symptom. You need to know why.

If you are using a basic scantool (code only) you need to look up the P0000 code.
I thought this an odd code. I looked it up in my Autotap unit and it does not exist. The lowest # P (powertrain) code I have listed is P0016.

A regular scan tool, won't scan transmission or body codes, mu Auto tap doesn't either. You need a Tech 2 from the dealer for transmission codes.

First some history. The first O2 sensor was introduced in 1976 on a Volvo. California vehicles got them next in 1980, then federal emission laws made O2 sensors virtually mandatory on all cars and light trucks built since 1981. And now that OBD-II regulations are here (1996 and newer vehicles), most vehicles now have multiple O2 sensors, some as many as four!
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust. The signal from the O2 sensor tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors affect the richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. Other sensors monitor these factors too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Problems with the O2 sensor can throw the whole system out of whack.
The computer uses the oxygen sensor's input to fine tune the fuel mixture for the best balance of power, economy and emissions. The engineering term for this type of operation is "closed loop" because the computer is using the O2 sensor's input to adjust the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which helps the catalytic converter operate at its best and keeps the average fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.
If no signal is received from the O2 sensor, like when a cold engine is first started (more on that in a minute) or the 02 sensor fails, the computer orders a steady, rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to fine tune the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. As you might have guessed, that will set a code and turn on your check engine light.
How does it work? The O2 sensor produces a voltage once it gets hot. The sensor compares how much oxygen is in the exhaust to the oxygen in outside air. The greater the difference, the higher the voltage reading.
If you ever replace an O2 sensor (and if you're a DIY'er this is something you will do eventually), its important to remember that the O2 sensor needs to "breath" outside air to work. So don't put any grease on the sensor because it could block this air flow.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen left in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
When the computer reads a rich signal from the O2 sensor it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs anywhere from 2 to 7 times a second at 2500 rpm on OBDII vehicles, depending on what type of fuel injection system they have.
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal. Many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.
Ok – that was a lot of info on what they do and how they work. The next thing to know is that trouble codes relating to O2 sensors are very common. But you really need investigate further before replacing an O2 sensor just because you got that trouble code. Armed with the information above on how often the O2 sensor "flips" back and forth and AutoTap or another scantool that allows you to monitor O2 sensor voltage, you can be certain whether the O2 sensor itself is really the problem. These sensors can be pricey, so don't just replace them the first time you see that trouble code!

The O2 sensors are expensive, diagnose what really is going on.

Posted on Aug 24, 2009

  • 14 Answers

SOURCE: Check Engine Light - O2 Sensor - bank 1 running lean

you should have three oxygen sensors on your car, one for each bank of three cylinders; should be towards the Y pipe on each exhaust manifold. Additionally you should have one on the exhaust pipe before the cadilitic converter.

Posted on Nov 20, 2008

CaptainKeyz
  • 202 Answers

SOURCE: have this code PO171, what does this mean.

Not sure that the O2 sensor was bad. It sounds like it was telling you that side of the exhaust system was picking up a lean mixure at the O2 sensor. The O2 sensor was doing its job by telling you the mixture changed. If the O2 sensor was bad, it would say low voltage range for that sensor, or open curcuit.
Now the car has had time to do its diagnostic sweep, it is telling you the same thing, but from a different sensor...the fuel mix is too lean. Air/spark/and fuel is what you need in the correct amounts. In your car, the ECM engine control module controls this by taking readings from all the sensors several hundred times a second.
Mechanically, you have a few options. Change the fuel filter...under the car on the passenger side. It is an easy job.
Change the fuel pressure regulator. This keeps the fuel at 4 BAR so the fuel injectors can do their job correctly. This job just needs a screw driver to pop off the clip that holds it in place. No hoses need to be removed. It is on top of the intake manifold, in-line with a metal fuel line and a rubber one.
The fuel injectors could be clogged up on one side of the engine. Run some fuel injector cleaner through a tank of gas. Also, listen for a constant ticking sound at each injector. If one sounds off PAR, then replace it.
The fuel pump may not be giving you enough power. However, the readings seem to be localized to one side, so I doubt it.
Start with the easiest thing, and see if it works. Also, you will need to get yourself a VAG scan tool. It is the only one with codes in it for all of your VW systems, not just engine codes like the ODB scanners. You can get a nice one for $50 on-line.
Good Luck!

Posted on Feb 24, 2010

  • 4088 Answers

SOURCE: New o2 sensor went bad in 1 day after being

The connections to the sensor is heater voltage which is +12 volts supplied by external (from the ECM) wiring from the battery, and the 0 to + 5 volt self-generated signal from the O2 sensor is an input to the ECM. Suggest that you find another mechanic that isn't intent on selling parts. Also suspect is the quality of the replacement O2 sensor. It would take a major disaster within the ECM to make an input connection turn into an output and would likely damage the O2 sensor almost immediately--not a day later. Hope this helps!

Posted on Jul 19, 2010

ZX2Fast
  • 547 Answers

SOURCE: check engine light code P0171

That code is a good O2 sensor reporting a lean condition. Lazy people and part stored just say it is the O2 sensor because they don't know how to fix it. Bank 1 is the side of the engine with the number one cylinder. That bank has a lean condition meaning it doesn't have enough fuel for the given amount of air. Since this is only affecting one side of the engine, that narrows down possible causes. Possible causes would a leaking intake gasket or a broken vacuum hose on that side. Since both sides get fuel from the same pump and rail, this wouldn't be a fuel pump or regulator issue. It can, however, be an indication of a bad fuel injector on that side of the engine. The MAP sensor also couldn't cause this code because, just like the fuel pump, that MAP sensor works for both sides of the engine and the code is only for one side. I would drive it for a couple more days to see if you get any additional codes that could help point you towards the actual problem. In the meantime, I would check for vacuum leaks (look at the rubber hoses and listen for high pitch hissing or sucking sounds.)

Posted on Mar 03, 2011

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I have a 1997 ford contour and the diagnostic tool reads left bank lean i was told that could be the fuel filter also my check engine light is on is this normal when the fuel filter needs changing?


It is not a fuel filter. If the left bank was lean the right would be also since the complete engine is fed from a single line to the injectors. Your O2 sensor is probably throwing the code to set the check engine light. The O2 sensors control the amount of fuel fed to the engine. Most Fords use an upstream and a down stream O2 sensor. From what you are describing I am betting your vehicle has way over 100,000 miles on it. The O2 sensors start going out after this point. They are subject to heat extremes and contamination from burned oil deposits and carbon and or fuel additives that you place into your fuel tank. If the code is reading or giving you a "lean" condition more than likely your upstream O2 sensor has bit the bullet finally.

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If you have access to a scan tool, check the signal voltage for the Bank 1,3 oxygen sensor with the engine at normal operating temperature. Is it stuck low currently? If so, increase RPM for a few seconds and see if it affects the reading. If it begins working with increased RPM, check for holes in the exhaust near the O2 sensor that may cause a false lean. If the exhaust pipe is intact, the sensor is sluggish, replace it. If the Bank 1,3 O2 sensor voltage reading remains low with increased RPM, unplug it and then observe the reading. It should have increased to about 0.5 volts or thereabouts. If it did, check for water intrusion or other connector problems. If none are found, replace the shorted O2 sensor.

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Went to get emissions done and my service engine light was on. Expectedly car failed. He told me I had to have Bank 2 senor 2 replaced and Bank one Sensor one replaced. Well I replaced the senors but...


Go to the nearest Autozone and do again a code scanning.If the code is reporting lean or rich mixture from O2 sensors, then the problem are not the sensors but something else.

Lean sensor is often caused by vacuum leak. Rich may be given by a defective sensor in air intake or by other causes, like an EGR valve stuck open. Check also fuel pressure to ensure that pump and pressure regulator are OK.

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Make sure you have all the vacuum leaks repaired.
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This sounds like an O2 sensor, there are 4 O2 sensors, 2 for each bank, 2 up stream by the exhaust manifold and 2 down stream by the catalytic converter. However the information you have given is limited, the explanation for the code seems to be missing some information. Ex: right bank sensor 1 to lean.

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P0174 Basically this means that an O2 sensor in bank 2 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 2 is generally the side of the engine that doesn't have cylinder #1.
P0140 Basically this means the an O2 sensor in Bank 1 detected a lean condition, same as mentioned above. Bank 1 is the side where cylinder #1 is.
In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling.

P1133 and P1153 indicate that the HO25 Sensor has gone bad. There the O2 sensor to the left and right bank at the exhaust manifold. When the engine first start's the PCM runs on a open loop and ignores the O2 readings till it reaches it's operating temperature. Once the operating temperature has been reached the diagnostic will only run once per ignition cycle and if the PCM detects a lesser then specified value, a DTC of P1133 or P1153 will come up.
Once the problem has been repaired, after 3 start cycle will reset the MIL light. If the O2 is just dirty and slow to heat up try cycling the ignition start 3 times and the light should go off if the O2 sensor does not fail.
Try cleaning the MAF first and see if the light comes off after a few ignition cycle, If the service light is still on you can always have Auto Zone clear the DTC codes and see if it comes back which then check the left and right bank wire harness to the O2 sensors. 
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Check Engine Light - O2 Sensor - bank 1 running lean


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