Understanding O2 signals and Fuel trim

I started writing this as a response to a post, but thought that it would be a good thread on it's own. Anyone that tries to figure out their own problems in their O2 sensors or try to figure out fuel trim should read this.

Reading and understanding fuel trim as well as the actual O2 sensor readings are almost an art. You should never expect bank 1 and bank 2 to coincide all the time, though they should be even on AVERAGE. Meaning that while the peaks and drops won't happen at the exact same time, they should have reasonably EVEN peaks and valleys. A static O2 sensor will read 450 millivolts. A lean signal will be low (around 200mv) and a rich signal will be high (around 800mv).

Fuel trim is a calculation based on the O2 sensor readings from the sensors at position 1. (Bank 1 sensor 1, Bank 2 sensor 1) Long term and short term are pretty much like they say. Short term fuel trim (STFT)is quick changes to the injector pulse to correct fuel mixture, and long term fuel trim (LTFT)is for continuous steady changes. When diagnosing fuel mixture problems I look more at the long term numbers. As long as LTFT is near zero then all is usually good. Quick changes are to be expected (i.e. STFT) because there are constant changes in the environment of the engine. Quick throttle closes, like when you take your foot off the gas pedal, will cause a quick spike in richness that STFT will correct almost immediately by going negative. There is no need to make a LTFT adjustment, because you are not having a continuous change in fuel mixture.

Now lets look at another situation. Let's say a small leak in a vacuum line. This small leak is picked up by the STFT and a small correction (+1 STFT) is made to add fuel. The O2 sensor shows that the correction did not last and more correction is needed. Again the STFT makes a change to add more fuel (+2 STFT)and a counter is started. When that counter reaches a predetermined point (this is done within a few seconds of course and is NORMALLY but not always a STFT value of +5) then the computer will increase the LTFT and will continue to monitor. This results in a +1 LTFT and a +5 STFT. This adjustment will continue up to a maximum of +25/+25 at which point the computer is unable to add more fuel. (Note that this also works backwards in the case of running rich. The difference being that the numbers would go negative, thus REDUCING fuel injector on-time)

MOST cars and trucks today will set a code when LTFT increases above 12, or below -12.

Don't try to diagnose ANY problems with O2 sensor readings or fuel trim numbers on a cold engine. Your engine will by design run slightly rich for a few minutes while warming up. Also the O2 sensor must be around 300 deg F before it will be accurate. Most of today's modern O2 sensors have a built in heater to speed this up. If your O2 sensor has more than 1 wire then it most likely is of the design with a heater. (This heater also is part of the OBDII diagnostic chain and will set it's own code if it has a failure)

 

That's a pretty good write-up. I would like to add that Toyota and Lexus can read higher and lower on the spectrum of fuel trim without necessarily setting a code. To me, they have a broader range than many other manufacturers. That is not to say that there may not be a rich or lean problem existing, but Toyota logic will not usually set a code unless the Trims are in the +/-30 percentile. Like said in the first post, normal readings are still +/-10. Check out this table from the TIS manual.

 

I vote for sticky on this thread. Great info!!

 

Yes, Toyota does have a +30/-30 threshold for setting the code. I should have been more specific on the Tacoma standard, but this post was meant to be generic, and not every year Tacoma nor every model of Toyota have the same threshold. This was meant as a more general information thread on how the systems work for all vehicles. Thanks for the input though!

 

Looks like you are the only one. lmao!

 

I would like a sticky too.

 

this is excellent info, thanks OP!

 

Most welcome. I am working on a thread about the dreaded P0420 code that I will post soon.

 

Good idea. I had that code come up a while ago. Cleared it, hasn't returned yet........

 

So this may be a stupid/broad question, but if my P0171 is running lean... what is the common fix? Is it a problem with the actual o2 sensor telling the computer to use less fuel, or is it typically a problem with something else?

Oh, and great thread, btw... +1 vote for sticky, so many members get these errors.

 

The upstream wide-band O2 sensor is actually seeing too much oxygen, and in turn telling the ecm to increase fuel to attempt compensating. P0171 can be caused by several issues. The big 2 on a Toyota truck can be a vacuum leak or a Mass Air Flow meter that has dirt or fuzz on the hot wire filaments. Other causes can be an AF sensor(wide-band O2) that is out of range, or even a fuel pump that may be getting weak. In your case, you could try replacing the fuel filter before the pump. Check for disconnected vacuum lines and pull out the MAF meter and look down inside of it with a light.

 

Changed my fuel filter last week. I've taken apart my intake system many times and there are no leaks (that I have seen).

 

Are you re-using gaskets?

 

Keep getting the code on my ultra gauge no light though and mpgs aren't effected any I'm just tired of it beeping ha

 

As mentioned, several possible causes of the lean code. In actuality the computer would be INCREASING fuel if the system is lean. Things such as vacuum leaks, bad O2 sensor, issues with MAF sensor, MAP sensor, fuel pressure (Weak pump, dirty filter) and even bad/dirty injectors can cause a lean code.

I can understand. The important thing you can look at on your Ultra is look at long term fuel trim and short term fuel trim. What are the numbers?

 

Ill get them when I go to class and post em up

 

Hmmm. Maybe this is why I'm getting such bad gas mileage? Too much fuel? I just changed my fuel filter and it was pretty clean. Nothing dirty came out. I didn't notice a huge difference once I changed it either. Maybe I'll run some stuff through to clean the injectors.

 

Idle
Srt -3 - 0
Lng 14.84

At 65
Srt -1.6 - 3.13 sometimes was at 0 ?
Lng 14.84

Not sure what the numbers are supposed to be

 

You will see STFT(Short term fuel trim) fluctuate a lot, as this is the computer fine tuning your fuel requirements. LTFT(long term fuel trim) is where the information is at. 15% LTFT is a typical indicator of a small vacuum leak, like a small hose, or a gasket. HOWEVER this can be misinterpreted if a MAF sensor is giving a faulty reading.

The formula used to check accuracy of your MAF sensor is complicated, but for those interested:

IMAP = RPM * MAP / IAT / 2 where RPM is engine speed in revolutions per minute, MAP (manifold absolute pressure) is measured in KPa, and IAT (intake air temperature) is measured in degrees Kelvin. This integated value can be converted into total air flow (grams) using the following formula: (grams of air) = (IMAP/60)*(Vol Eff/100)*(Eng Disp)*(MM Air)/(R) where R is 8.314 J/°K/mole and the average molecular mass of air is 28.97 g/mole. Note that in the above formula the volumetric efficiency of the engine is measured in percent and the displacement is in liters.
(Copied from http://www.lightner.net/obd2guru/IMAP_AFcalc.html)

You can use this formula to test if your MAF sensor is accurate. It can also be used to verify a vacuum leak, which can result in a lean condition, and P0171/P0172 codes. I have actually used this formula to diagnose an inaccurate MAF sensor in a Ford that would hesitate slightly, and show lean, but the customer had recently replaced all of their O2 sensors.

 

what if im not getting any of those codes ?? Geuss ill give my MAF a good cleaning