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.


Testing Oxygen Sensors

There are a few different tests for oxygen sensors (also known as lambda sensors), some of which can be run without dedicated tools. The most effective tests tend to be done under normal operating conditions, on a sensor installed on an engine system, though there are some tests which can be done off-vehicle. You can test oxygen sensors with the following tools:

Caution: Be sure to follow the oxygen sensor manufacturer’s precautions when testing, as well as the tool manufacturer’s directions, and read the vehicle (or other system) service manual before doing any test. Oxygen sensors get very hot when in use, be careful!


How to Use a Battery Tester

Most battery testers have two clips (or test leads), you first attach these to the battery’s terminals (connectors), then you press a button (or switch), and a few seconds later, the tester displays or prints a result indicating the battery’s condition. The details of you use a battery tester depends on what kind of battery you have, the electrical system it is connected to, and which tester you plan to use. Some battery testers require disconnecting the battery from the vehicle, while others have different limitations. Most battery testers only work on one, or a few different types of batteries. We describe how to use five different battery testers below:

  1. Carbon Pile Battery Testers
  2. Hand-Held Battery Load Testers
  3. Battery Hydrometry Testers
  4. Battery Conductance Testers
  5. Multimeters

Caution: Be careful to follow the battery manufacturer’s precautions when testing, as well as the tester manufacturer’s directions, and read the vehicle (or other system) manual before doing any test, or even disconnecting the battery.