In modern cars with OBD-II systems, the front oxygen sensor measures the amount of oxygen in the exhaust gas and provides feedback to the engine computer ECM (or PCM), to see if the air-fuel mixture in the engine is poor (too much air and too little fuel) or rich (too much fuel and too little air).
The ECM (or PCM) adjusts the fuel supply accordingly to keep the air/fuel ratio at its optimum, which is 14.7/1 (14.7 parts air to 1 part fuel).
This process goes on continuously: When the oxygen sensor detects too much oxygen, the ECM (or PCM) will assume that the air/fuel mixture is thin and add more fuel. When there is too little oxygen in the exhaust gas (condensed air/fuel mixture), the ECM (or PCM) will reduce the amount of fuel. In technical language, this adjustment is called Fuel Trim, basically understood as fuel compensation.
Short term fuel trim (STFT)
When the front oxygen sensor is warmed up after a cold start, the engine computer ECM (or PCM) will begin to adjust the air/fuel ratio between thin and dense (poor and rich). This cycle happens about once or twice per second. This air/fuel ratio adjustment is called Short term fuel trim (STFT). Using a diagnostic scanner will show STFT as a percentage with “-” or “+” in Live Data mode. For example, if your car has a small vacuum leak, you’ll see the STFT increase to +10-15 % at idle and drop to +3-5% at higher speeds.
Long-term fuel trim (LTFT)
There is also a long-term fuel trim (LTFT), which is the long-term fuel trim of the base air/fuel ratio. For example, if over time the engine gradually develops a small vacuum leak, it causes the engine to run more lean (more air and less fuel). In the long run, the engine computer ECM (or PCM) compensates for this by adjusting the long-term fuel trim (LTFT) to inject more fuel. Long-term fuel trim is also displayed as a percentage with a “-” or “+” sign in a diagnostic scan tool. For example, a long-term fuel trim (LTFT) of +5% means that the ECM (or PCM) added some fuel, even though 5% is considered within the normal range.