Fuel Trim During a Real Drive: An OBD2 STFT and LTFT Case Study

What STFT and LTFT represent

Short-term fuel trim (STFT) is the engine computer's relatively immediate fuel correction. Long-term fuel trim (LTFT) represents a learned correction accumulated over a longer period. Positive and negative values indicate corrections in opposite directions; interpretation depends on operating conditions and the vehicle's control strategy.

For this reason, an isolated fuel-trim value should not automatically be treated as a diagnosis. The interesting question is often what was the engine doing when the value changed, and did the behavior persist?

1. Fuel trim across the complete drive

Figure 1. All recorded STFT and LTFT values plotted against elapsed recording time. Negative values in the CSV export are included.

The recording lasted about 8.2 minutes. It contains 29 STFT readings and 26 LTFT readings. STFT ranged from -18.8% to +7.0%, while LTFT ranged from -6.2% to +6.2%.

2. The −18.8% short-term corrections

STFT reached its minimum recorded value three times. The table shows nearby operating measurements. Because parameters are polled sequentially, these are contextual samples rather than perfectly simultaneous readings.

The first −18.8% sample at 16:30:09 was followed by positive STFT readings shortly afterward. The same minimum appears again at 16:33:09 and 16:36:55. That repeated but nonpersistent behavior is exactly the kind of pattern that can disappear when live data is viewed only as a snapshot.

3. Put the fuel trim in operating context

Figure 2. Engine RPM during the same recording.

Figure 3. Calculated engine load during the same recording.

The session also contains speed, throttle position, MAP, coolant temperature and ignition timing. These signals show that fuel-trim readings were collected while engine conditions were changing, rather than at one fixed operating state.

4. What the data does — and does not — show

The STFT excursions are values present in this recording, but this one session does not establish a vacuum leak, rich condition, fuel-delivery fault, sensor failure or another specific diagnosis. Fuel trim changes with operating conditions and control strategy, and sequential polling limits exact moment-by-moment correlation among parameters.

What the recording demonstrates is why context matters. A short-term correction can change markedly over a small part of a drive, while LTFT can present a different, more restrained pattern.

5. What a live snapshot could miss

If a conventional scanner were checked at one instant during this drive, STFT might have appeared near zero, positive, moderately negative, or at −18.8%. Each could accurately describe that sample, but none alone describes the entire session.

This is the purpose of continuous car data: retain the sequence so patterns, changes and relationships can be examined after the drive rather than requiring the driver to watch changing numbers on a screen.

How this recording was made

Car Health Data recorded supported OBD-II parameters through a compatible Bluetooth adapter and mobile app. The export covers 16:29:00–16:37:15 on October 17, 2026, approximately 8.2 minutes, and contains 276 rows across ten metrics. Parameters are requested sequentially, so graphs should be interpreted as trends rather than millisecond-level synchronized measurements.

This is one vehicle and one real-world drive. The purpose is to demonstrate what can be learned from retaining live OBD2 data over time, not to claim that the observed values establish a mechanical fault.

Technical note: This case study reports the supplied measurements as recorded. It is educational information, not a substitute for mechanical inspection or professional diagnosis. Original session identifier: 1F8A1008-4682-4972-8F98-40711314DA87.