Long Term Fuel Trim and Short Trim Fuel Trim

What is Fuel Trim?

Fuel trims are what engine control system use to compensate for all problems relating to air-fuel ratios (known as λ or lambda,) and combustion.

The Electronic Control Module (ECM) or Powertrain Control Module (PCM) uses a few sensors to determine how much air is flowing into the engine. It then uses the airflow data, along with a target air-fuel ratio (λ or lambda) to calculate how much fuel it should inject. The ECM or PCM then uses oxygen sensor readings to find out what the actual air-fuel ratio was, and ‘trims’ the fuel quantities based on this ‘real’ air-fuel ratio.

If the PCM/ECM detects that the calculated quantity of fuel needed is too much or too little (with the oxygen sensor), the PCM/ECM will add or subtract fuel to ‘trim’ the quantity of fuel and optimize the combustion. The difference between the calculated and the actual quantity of fuel being delivered to the cylinders is what is called “fuel trim”. Fuel trim is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).

fuel trim flowchart

Fuel trim is related to a variety of sensors and variables.