How to Read Fuel Trim Open The fuel trim value is read by plugging a scan tool into the OBD II diagnostic connector located under the instrument panel (on the drivers side near the steering column). When the key is turned on, the scan tool will initialize and start to communicate with the vehicle�s onboard computer. Depending on the tool and the vehicle, it may be necessary to enter the vehicle year, make, model and engine VIN code before the scan tool can read the data. The engine must be started and running to read the fuel trim information. Depending on the scan tool and how its menu options are set up, you choose the option that allows you to read system live data. This will display a long list of sensor outputs and other readings called PIDs (Parameter IDs). On this list will be two fuel trim values for inline four and six cylinder engines, and four fuel trim values for V6 and V8 engines (one pair for each cylinder bank). There are two types of fuel trim values shown: Short Term Fuel Trim (STFT) is what the engine computer is doing to the fuel mixture right now. This value changes rapidly and can bounce around quite a bit depending on engine load, speed, temperature and other operating conditions). Values normally range from negative 10 percent to positive 10 percent, though the readings may jump as much as 25 percent or more in either direction. Long Term Fuel Trim (LTFT) is a longer term average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of time. This value is a more accurate indicator of how the fuel mixture is being corrected to compensate for changes in the air/fuel ratio that are occurring inside the engine.
What Fuel Trim Values Mean POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engine�s air/fuel mixture is running too lean. NEGATIVE (-) fuel trim values mean the engine computer is subtracting fuel (decreasing the pulse width or on-time of the fuel injectors) to reduce the amount of fuel injected into the engine. This is done to LEAN out the fuel mixture to compensate for what it perceives as a rich running condition. Remember, all this is based on what the oxygen sensors are telling the engine computer. If the O2 sensors indicate LEAN, the computer adds fuel and generates a POSITIVE fuel trim value. If the O2 sensors are reading RICH, the computer compensates by subtracting fuel and generates a NEGATIVE fuel trim value. By reading the STFT and LTFT fuel trim values on a scan tool while your engine is running, you can tell if the air/fuel mixture is running rich (negative fuel trim percentages) or lean (positive fuel trim percentages). What Fuel Trim Values Should Be Ideally, the STFT and LTFT should be within a few percentage points of zero when the engine is idling or being held at a steady RPM. Remember, STFT can bounce around quite a bit as when you suddenly snap open the throttle or decelerate. But LTFT can tell you if the average fuel/mixture is running rich or lean. Good LTFT values should be as close to zero as possible, though they can range from 5 to 8 percent depending on the condition of the engine. If the LTFT is getting up around 10 percent or higher, it usually indicates a problem that needs to be diagnosed.
Some possible causes of LEAN fuel mixtures include: Air or vacuum leaks in the intake manifold, near the throttle body or at vacuum hose connections.
Weak fuel pump that is not generating enough pressure or volume Fuel line restrictions (like a pinches hose or plugged filter) A weak fuel pressure regulator that is not maintaining adequate fuel pressure Air leaks in the PCV plumbing Dirty MAF (Mass Airflow) sensor that is under reading airflow into the engine Dirty or dead fuel injectors Ignition misfire (a fouled spark plug, weak ignition coil or bad plug wire that causes a misfire allows unburned oxygen to pass into the exhaust and fool the O2 sensors) Compression leaks (bad exhaust valve that allows unburned oxygen into exhaust and fools O2 sensors) Exhaust manifold crack or gasket leak (allows unburned air into exhaust and fools O2 sensors) Bad O2 sensor (signal shorted to ground so the sensor reads lean all the time) Some possible causes of RICH fuel mixtures include: Leaky fuel injector Excessive fuel pressure due to bad fuel pressure regulator or restricted fuel return line Extremely dirty air filter or restrictions in air intake system Exhaust restrictions (clogged converter, crushed exhaust pipe or plugged muffler) Bad O2 sensor (output shorted to voltage so it reads RICH all the time)