What Is Feedback Knock Correction on an Accessport Datalog?
Feedback knock correction is timing the ECU pulls out, right now, because its knock sensor heard something it did not like. It shows up in a datalog as a negative number of degrees.
What it is
On Subaru and other COBB-supported cars, the ECU listens for knock with a sensor on the block. When it hears knock in a cylinder, it removes ignition timing for a moment, then puts it back. That live adjustment is feedback knock correction, often shown as Feedback Knock or FBKC in degrees. Zero means no correction. A negative value, such as −2.1°, means the ECU pulled that much timing.
What is usually normal
A single small correction as the throttle moves, as boost first builds, or right after a gear change is common, and is often just noise. Many healthy logs show an occasional correction that never repeats.
What matters is how big, how often, and under what load.
When it deserves attention
- Repeated corrections in the same part of a pull.
- Larger corrections, especially during sustained high load or wide-open throttle.
- Corrections that line up with something else going wrong, such as a lean mixture, falling fuel pressure or high intake temperatures.
APDataview's own rule looks at size, repetition and operating state together. By default it asks for a closer look at corrections of about 2° or more that repeat, and treats larger corrections under sustained high load as more serious. Those are APDataview's settings, not an industry standard, and a tuner can change them.
What to check next
- Look at DAM in the same log. If it has dropped below 1.0, the ECU has been learning from knock, not just reacting to it.
- Look at air/fuel ratio and fuel trims at the same moment (fuel trims, AFR).
- Check fuel quality, intake temperature, and spark plug condition.
- Log the same pull again and see whether the corrections repeat.
Share the log with your tuner. Knock is the thing to take seriously: a log is evidence, not a verdict.
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 any feedback knock correction bad?
Not necessarily. Small, isolated corrections are common. Repeated or large corrections under load are what to investigate.
What is the difference between feedback and fine knock learn?
Feedback knock is the live correction. Fine knock learn is timing the ECU has learned to remove in a given RPM and load region because of past knock.
Can APDataview tell me what is causing the knock?
No. It shows where the corrections happened and what else was going on at that moment. The cause has to be found by checking fuel, temperatures, the tune and the engine.
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.