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)