STFT (Short Term Fuel Trim) Meaning in OBD2 Diagnostics

STFT (Short Term Fuel Trim) is an important Live Data parameter in OBD2 diagnostics. It shows the immediate fuel adjustments made by the ECU to maintain the correct air-fuel mixture.

What Does STFT Mean?

The ECU monitors oxygen sensor feedback and continuously adjusts fuel injection. Short Term Fuel Trim represents the quick corrections the ECU makes in response to changing engine conditions.

STFT values are displayed as a percentage (%) in most diagnostic scanners.

How Does STFT Work?

When the oxygen sensor detects a lean condition (too much air or not enough fuel), the ECU increases fuel delivery and STFT becomes positive.

When the oxygen sensor detects a rich condition (too much fuel), the ECU reduces fuel delivery and STFT becomes negative.

Understanding STFT Values

STFT Reading	Meaning
0% to ±5%	Normal fuel correction
+10% to +20%	ECU is adding fuel (possible lean condition)
Above +20%	Possible air leak, low fuel pressure, or sensor issue
-10% to -20%	ECU is removing fuel (possible rich condition)

Common Causes of High Positive STFT

• Vacuum leak
• Unmetered air entering the engine
• Low fuel pressure
• Dirty fuel injectors
• Faulty MAF sensor
• Exhaust leak before oxygen sensor

Common Causes of Negative STFT

• Leaking fuel injector
• High fuel pressure
• Incorrect MAF reading
• Faulty oxygen sensor
• Excess fuel delivery

How to Diagnose STFT Using Live Data

1. Connect an OBD2 scanner.
2. Open Live Data and select STFT values.
3. Observe readings at idle and higher RPM.
4. Compare STFT between engine operating conditions.
5. Check related sensors such as MAF, MAP, and O2 sensors.

Example of STFT Diagnosis

High STFT at Idle Only:
If STFT is high at idle but improves at higher RPM, a vacuum leak may be the cause.

High STFT at All RPM:
If STFT remains high during all conditions, possible causes include low fuel pressure or incorrect air measurement.

STFT vs LTFT

Parameter	Function
STFT	Immediate short-term fuel corrections
LTFT	Long-term learned fuel adjustments stored by ECU

FAQ

Is a small STFT change normal?

Yes. The ECU constantly adjusts fuel and small changes are expected during normal operation.

Can STFT diagnose vacuum leaks?

Yes. STFT patterns at idle and under load can provide clues about possible air leaks.

Related Topics:
OBD2 Live Data | LTFT | Fuel Trim Diagnosis | O2 Sensor Testing