"P0151 = Left side O2 Rich" Fuel or Oxygen?.

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JohnDee68
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"P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by JohnDee68 »

Hi all,
I am learning and trying to get me head around this, as there are conflicting information online as to what is going on to cause a P0151 error.

Can someone please actually explain in layman's terms what actually is going on, presuming it's not a faulty sensor or wiring but due to fuel quantities being supplied to the combustion chambers by injectors as I am playing with the Tune and need to understand what actually is happening.

1. Online of some "professional mechanics" sites, they say a generic code of P0151 is a low voltage bellow 200mv on the O2 sensor of Bank 2 sensor 1 left hand side .
2. Others say that the code P0151 is a Excess oxygen detected by the O2 sensor on that same O2 sensor location.
3. My diagnosis software says "P0151 = Left side O2 Rich." for the error. I presume it is referring to Rich as in too much fuel ?

I am suspecting the code is generated from too little fuel in the combustion chambers of the Left side, (why the sites say too much Oxygen is still beyond me) but I can't see how that would be called a Rich condition? From my current understanding a Rich mixture is when an O2 sensor reads above 500mv. I was told to understand that above 500mv is Rich, bellow 500mv is Lean, so I am confused.

Any help to explain this is appreciated.
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by ironduke »

From gm service information for a 21 silverado 5.3(current vehicle I'm working on)
DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1
Conditions for Setting the DTC : Heated Oxygen Sensor = Less than 50 mV — For greater than 50 s

When the engine is started, the ECM operates in an open loop mode, ignoring the HO2S signal voltage while calculating theair to fuel ratio. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 40 to 1050 mV. Once sufficient HO2S voltage fluctuation is observed by the ECM, closed loop is entered. The control module uses the HO2S voltage to determine the air to fuel ratio. An HO2S voltage that increases toward 1000 mV indicates a rich fuel mixture. An HO2S voltage that decreases toward 0 mV indicates a lean fuel mixture because it is seeing an excess of Oxygen. In addition to the hot exhaust gas, heated oxygen sensors have a resistor heating element to minimize the time required for the sensor to reach operating temperature. The ECM monitors the temperature and turns the heater on as necessary.

In layman's terms, O2 sensor doesn't see shit about fuel.. doesn't care, doesn't see it, has no use for it.. It sees Oxygen.. it compares O2 levels inside the exhaust to the O2 level outside the exhaust. Supposedly the wires allow O2 to travel along the wire from the ecm for the outside O2 sampling? lol.. no idea if that's true, I just know I've been told to never repair O2 wiring and instead replace the sensor..

The O2 sensor voltage is too low for too long a period of time.. This is because the O2 level in the exhaust has close to the same amount of O2 outside the exhaust.. This can be caused by a lean condition that cannot be rectified by fuel trims, exhaust leak letting in outside air in front of or near the O2 sensor, or a bad sensor..

Usually it's a bad sensor with that code. I look at the post cat O2 sensor on that bank(Bank2 Sensor2) and if it's not showing the same very low voltage reading as the precat than I lean towards a bad sensor.

You can enrich it with propane gas or even carb clean or brake clean and look for a voltage spike.. no spike means O2 sensor or wiring is not correct..

Course if it's a dead misfire all bets are off as that dead cylinder is just an air compressor at that point pushing O2 into the exhaust and will also cause a low O2..

Did that help??
JohnDee68
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by JohnDee68 »

Thanks ironduke, that did help towards my understanding.
I then come back to the basic of understanding for actually what happens in a combustion. It would seem then that Air and Fuel requires a set range of ratio to burn efficiently of both fuel and oxygen. Ideally on a perfect burn, the whole of the oxygen would be burnt from what ever that fuel / oxygen ratio is at the time. So when there isn't enough fuel in the mix, then there is left over oxygen which is expelled after combustion. Which will result in the O2 sensor sensing a higher oxygen level, which is actually meaning there was less fuel in the combustion at that time which is a Lean condition. Which when this happens for a extended period of time, the ECU will likely throw up the code. Which is why the code can come on and off at times.

If I am correct in that, then it would also explain why there is some sites saying that the P0151 code is a Rich Oxygen condition, where others say it is a Lean mixture condition. (only after ruling out faulty sensors or wiring of course). They are basically saying the same result, just in different ways.

Am I correct in my thought of logic ?
If I am correct in my logic, then it could be a good chance a bad fuel injectors (restricted or failing) could also be a cause to throw up the code at times?
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by hsv08 »

They say "not" to repair 02 wiring because the ecu signal from the sensors are in millivolts, by making a repair, you can induce resistance. Which may not affect anything in the 1-12volt range, but when you depend on voltages in the millivolts range, any resistance in the circuit can skew readings.

Generally speaking, when the sensor fails, the sensor value reads low millivolts, this causes the pcm to think its lean and to fuel up that bank.

Exhaust leaks are common to cause a perceived lean mix, but if it's pulling a P0151 and you checked you have 450mv at the pin A of the O2 loom, then the sensor will be buggered.
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by hsv08 »

I think we need to start a "johndee68" forum that you can bombard with all your questions haha.
JohnDee68
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by JohnDee68 »

hsv08 wrote:I think we need to start a "johndee68" forum that you can bombard with all your questions haha.
I take that with a good laugh. I was always told, when not sure ask questions. So I hope I am not being a pain. Maybe too much time on my hands?

I agree with the don't repair the wiring for sensors, though I would presume as I have seen more actual connectors cause voltage drops than many more solder joints. But it is possible and a good thought to keep in mind.

EDIT: I also haven't said that the sensor isn't in my case under suspicion as being at fault. I am suspecting it is due to one or more injectors being restricted. As the error has only happened while trying to lean the injectors up through TunerPro. (I am trialing adjustments). I believe that there are one, possibly two injectors on the left hand side which are causing slight higher fuel injections. This has become noticeable since I reduced the Injector Base Rate to closer to the calculated value (550MSEC/GRAM) using the service manual specs of my original BOSCH injectors. I think there are 4 of my original injectors, 1 of which is on the left hand side. The other two on the left side are from a unknown motor and possibly bigger flow rated, but were bench tested and became as the better choice when the motor was rebuilt. I am still going to do a few more checks on Air Flow rates to confirm it is fuel issues not airflow issues.
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by ironduke »

What are your fuel trims running at? It's possible it's 1 injector but usually the voltage has to go below a hard limit that it's never supposed and this is for a certain time period, I would think the other injectors that are firing ok would spike the O2 above the hard limit long enough to reset the counter for the code test..
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by JohnDee68 »

ironduke wrote:What are your fuel trims running at? It's possible it's 1 injector but usually the voltage has to go below a hard limit that it's never supposed and this is for a certain time period, I would think the other injectors that are firing ok would spike the O2 above the hard limit long enough to reset the counter for the code test..
Thanks for replying ironduke, I haven't managed to do a log yet since the error had occurred. I plan to do more logs next week. Keeping in mind the error only happen twice in the two days, but self cleared the Check Engine light while driving both times it appeared durin different driving times. I think it only appeared during decel while also at 0% throttle around the 1800RPM and 60kmph downwards. So I am suspecting either the setting for the IAC or injectors are of concern. Other than that the car is running best it has for some months. Though I am a perfectionist and know I can get it better. I am aiming for maximum economy over power. Something which i don't think many or any one else on here is wanting and I understand that to.
Day prior to the error being triggered, I played with and changed the Base injector rate to 550MSEC/Gram as setting to the 560 run rough, where the 336.xx default in the enhanced xdf didn't allow me to get anywhere the economy i know the engine can get. While the value also is too low for my injectors, well at least 3 of them anyway. So I know the issue is to do with fuel amounts in the cylinders. My thought is that as I had got the injectors down to their minimum say of fuel injection, the variance of my injectors and the unknown couple injectors is showing up more so enough to flag the code. Where when they are pumping in more fuel at the 336.xx setting, it wasn't showing the imbalance as much.
I just don't want to fork out on new injectors until I can be 100% (or close to ) sure they are giving me the issues. So I would like to try Airflow adjustments next and see if I get either more error codes, or not or an increase in manifold vacuum under driving conditions. As they are slightly lower than desired still. Then in my logic it will help to confirm an injector imbalance issue, I think.
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by delcowizzid »

If they are stock injectors don't change the size value its fine as is my vx has 540000kms on the stock injectors and still cruises at 6-7L/100km on a flat road it's the getting up to speed that causes the consumption of you look at the instant fuel usage figures on the dash they go extremely high even with very little throttle driving gentle is key. There's a little to be saved with the lean cruise delay settings so it comes on faster. My car struggles to maintain operating temp over winter the radiator works to well it causes some extra consumption. Mine has the o2s disabled and just runs open loop all the time average 12l/100 daily driving over a hill with a 60% grade up both sides at 80kmh twice a day. On the open road 9-10l/100 nothing more than some shift point changes to make it feel sportier and 2 degrees added to the whole timing table
If Its Got Gas Or Ass Count Me In.if it cant be fixed with a hammer you have an electrical problem
JohnDee68
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Re: "P0151 = Left side O2 Rich" Fuel or Oxygen?.

Post by JohnDee68 »

delcowizzid wrote:If they are stock injectors don't change the size value its fine as is my vx has 540000kms on the stock injectors and still cruises at 6-7L/100km on a flat road it's the getting up to speed that causes the consumption of you look at the instant fuel usage figures on the dash they go extremely high even with very little throttle driving gentle is key. There's a little to be saved with the lean cruise delay settings so it comes on faster. My car struggles to maintain operating temp over winter the radiator works to well it causes some extra consumption. Mine has the o2s disabled and just runs open loop all the time average 12l/100 daily driving over a hill with a 60% grade up both sides at 80kmh twice a day. On the open road 9-10l/100 nothing more than some shift point changes to make it feel sportier and 2 degrees added to the whole timing table
Thanks Delcowizzid, I would agree with you that if they were "stock" injectors and the Tunerpro was calculated for the 361.33MSEC/Gram on a "stock" injector specs. But i have found that the actual "stock" injector specs likely useed initially may have been wrong. Here are what i have found:
BOSCH# \LBS/HR\CC/MIN\GRAMS\PSI\BAR\Resistance\Source
0-280-155-777\14.25\149.8\107.7\43.5\3\- \From actual Service Manual "stock" injectors.
0-280-155-777\21.44\219.85\162\3.5\12\From Forum
0-280-155-777\21.44\219.85\162\3.5\12\From internet
0-280-150-777\26.35\276.9\199.2\-\-\-\From internet
It is possible that BOSCH might have changed their specifications over time as well for the same part number item.

So with the Tunerpro using injector calculator, the "stock" injectors works out that it should be around 557.137 MSEC/GRAM for the "stock" injectors base rate, not 361.33MSEC/GRAM.
From my understanding at the 361.33 it is going to dump more fuel in to the cylinders when using the "stock" injectors that run at 149.8 CC/min where the TunerPro is set to run with injector values of the more closer to 276 CC/Min injectors. I am finding that the slight change in making the change, has likely pinpointed a miss match in the injectors I currently have fitted. As at least 2 on the left aren't likely "stock" injectors. I plan to test the flow rates of the injectors, hopefully in the next few weeks to confirm their cc/mm values. I will then know if i am correct or not in my thinking. I also feel that this could be the main contributing factor to performance drop and up to 60% down in economy from what it is meant to be.
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