Fuel Trim Stuck at −25%: What an Accessport Datalog Is Telling You
A fuel correction pinned at −25% means the ECU is removing as much fuel as it is allowed to and it still is not enough to reach its target mixture.
What fuel correction is
Between the engine's fueling table and the oxygen or wideband sensor, the ECU trims fuel to hit its target air/fuel ratio. In an Accessport log this is usually A/F Correction (short term) and A/F Learning (long term), shown as percentages. Positive means adding fuel, negative means removing it.
Why a pinned value is a flag
Trims move up and down when things are healthy. A trim that sits at exactly −25% for minutes at a time is not adjusting, it is hitting a limit. That says the base fueling and the measured mixture disagree by a lot in those conditions.
In one real log reviewed with APDataview, the correction reached −25.0% and stayed at exactly −25.00% for about 151 seconds while the measured air/fuel ratio was still richer than commanded. That combination is what a pinned limit looks like.
Common causes (general information)
- Fueling scaling in the tune that does not match the installed intake, MAF housing or injectors.
- Fuel pressure higher than the tune expects.
- A leaking or over-fuelling injector.
- An exhaust leak ahead of the front air/fuel sensor, or a sensor reading inaccurately.
- Operating conditions the log did not capture, such as warm-up enrichment.
These are things worth ruling out. APDataview does not decide the cause from a log.
What to check next
- Log again fully warm, with coolant temperature included, and compare.
- Check the long-term learning value as well: if it is stuck at 0.00% for the whole log, learning may not be active.
- Share the log with your tuner.
APDataview's default rule flags a correction beyond about 20% that lasts several seconds. That is its own setting and a tuner can change it.
Check your own log
Upload a COBB Accessport log to APDataview and see whether this shows up in it, with the exact stretch of the log behind every finding. It is free, and the same log always gives the same result.
Check my log free →Common questions
Is −25% the maximum?
On many COBB-supported calibrations the correction is limited to about ±25%, which is why it reads as a hard stop. Check your own tune.
Is a negative trim always bad?
No. Small trims are normal. Large trims that persist, and especially trims pinned at a limit, are worth investigating.
Does a pinned trim mean the engine is damaged?
Not by itself. It means the fueling and the sensor disagree. A rich mixture is usually less urgent than a lean one, but it still needs explaining.
Related guides
General information only. APDataview reads what the ECU recorded and cannot see your engine, fuel or hardware. It is another tool in your toolbox, not a substitute for professional diagnosis or tuning.