[mustang67408]

Feedback from the 02 or A/F sensor(s) influence short term fuel trim and then in turn short term fuel trim influences long term fuel trim.
Short term values are not stored in memory and are lost when the ignition is turned off. Long term values are stored in memory, because they are part of the basic injection duration. Long term values affect injection duration in OPEN and CLOSED loop because they are used to calculate basic injection duration
It is important to remember that ACTUAL fuel trim is opposite of the actual DTC code stored.
A system to lean with code of P0171 will mean the ecu is making a RICH correction.

FUEL TRIM:
As things change over time the ECU needs a way to adjust the injection duration and improve drivability and emmisions performance. Fuel Trim is a program in the ECU designed to compensate for these changes.
Fuel trim allows the ECU to LEARN and adjust the injection time quickly back to "NORMAL"
SHORT TERM FUEL: Instant responce and correction
SHort FT is based on the 02/-A/F sensors and therefore only functions in closed loop. SHort FT responds rapidly to changes in the O2-A/F. If Short FT is varying close to 0% little or no correction is needed. When SHORT FT is positive, the ECU has added fuel by increasing injection duration. A negative% means it is subtracting fuel by decreasing duration.
The SHORT FT value is a TEMPORARY value and not stored.
SHORT TERM FT is used to modify LONG TERM FUEL TRIM. When the SHORT FT remiains HIGHER or LOWER than expected, the ECU will add or subtract this from the LONG TERM VALUE.
SHORT TERM FT is a temporary addition or subtraction of fuel to basic injection duration


LONG TERM FUEL:
Long FT is stored in memory because it is part of the basic injection duration calculation. The ECU uses the SHORT FT to modify the LONG FT
Long FT does not react rapidly to sudden changes, it only changes when the ECU decides to use the SHORT FT value to modify it. It is run in open and closed loop and is stored in memory.
A positive % indicates the ECU is adding fuel and a negative% indicates its removing fuel.
LONG TERM FT is part of basic fuel trim and stored in memory as a guideline.