What positive and negative per-cylinder corrections mean, which conditions matter and how to separate a stuck-open injector from lost compression.
Injector corrections show how much fuel the ECU adds or removes on each cylinder to balance the engine at idle. Learn the conditions for reading them, how to compare cylinders and how to separate a faulty injector from low compression.
Per-cylinder fuel corrections are among the most photographed values on a scan tool, and the question "are my injectors good?" rarely gets a correct answer from a single photo. The ECU adjusts fuel quantity per cylinder to keep idle speed steady, so the number describes an imbalance, not a specific part. This guide explains what the figure measures, which conditions matter and how to separate an injector from a compression problem.
Original editorial schematic, illustrative only — not a real scan-tool screenshot and not a substitute for engine-specific procedures.
Judge corrections only at idle, with the engine warm, no active faults and all consumers switched off. A large deviation on one cylinder calls for a return-flow test and a compression check before ordering an injector.
Common-rail diesel injector — component photograph Photo: Panoha · CC BY-SA 3.0 · Resized and compressed to WebP; image content unchanged.At idle, the ECU compares each cylinder's contribution to crankshaft rotation, measured through speed variations, and adjusts the injected quantity until the engine runs evenly. The displayed value is that adjustment: how much fuel the ECU adds to or removes from that cylinder.
The sign matters more than an isolated number. A positive value means the ECU adds fuel because that cylinder contributes less than the others. A negative value means it cuts fuel because the cylinder contributes more. Either can come from an injector, but also from compression, valve timing or sealing.
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2. What a per-cylinder correction actually represents| What the tool shows | Careful reading | What it does not prove |
|---|---|---|
| Positive value on one cylinder | That cylinder receives extra fuel to hold idle speed | That the injector is clogged: low compression produces the same effect |
| Negative value on one cylinder | The ECU reduces fuel on the cylinder producing more | That the injector drips: check return flow and rail pressure |
| Zero or missing value | The cylinder is balanced, or the parameter is unavailable | That the system is healthy: confirm the value is real and not just unsupported |
| All values close together | Good balance between cylinders at that moment | The absence of other faults: it does not cover pressure, return flow or compression |
On VAG vehicles, block 013 usually shows per-cylinder idle stabilisation in mg/stroke, while block 023 shows injector command timing deviations in a different unit. These are distinct quantities: do not mix them, and do not compare a 013 from one engine with a 023 from another.
In workshop practice, many common-rail engines are discussed with deviations around ±1 mg/stroke as a quiet zone and justified concern around ±2 mg/stroke on a single cylinder. These are discussion references, not official limits: the manufacturer publishes thresholds for the exact engine code, and some engines run correctly with larger deviations.
On the 1.5 dCi the principle is the same, but the unit and thresholds differ and are read with Renault-compatible equipment rather than a generic emissions scanner. Use the values from the documentation for your engine code, not figures copied from a forum thread about another version.
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4. Measuring blocks and reference points| Application | Where it is usually read | Typical unit | Watch out |
|---|---|---|---|
| VAG, block 013 | Per-cylinder idle stabilisation | mg/stroke | Compare cylinders with each other, not with another engine |
| VAG, block 023 | Injector command timing deviation | its own unit, often ms | It is not the same quantity as block 013 |
| Renault 1.5 dCi | Per-cylinder quantity corrections with compatible equipment | mm³ or mg, depending on the tool | Thresholds depend on the software version |
| Generic OBD scanner | Usually does not expose per-cylinder corrections | no useful value | Do not read a missing parameter as a good value |
An injector that stays open or drips delivers too much fuel: black smoke, a strong diesel smell, knocking at idle and sometimes unstable rail pressure. The ECU tends to reduce quantity on that cylinder, so expect a negative correction. Confirmation does not come from the scan tool but from a return-flow measurement and a bench test.
A clogged or worn injector delivers too little, and the ECU compensates positively. This is the trap: low compression on the same cylinder produces exactly the same sign. The difference is settled by a compression test and, if needed, a cylinder leak-down test, before removing any injector.
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5. Stuck-open injector, clogged injector or lost compression| Situation | Common signs | What confirms it |
|---|---|---|
| Stuck-open or dripping injector | Black smoke, diesel smell, knocking, rough idle | Return-flow test, bench test, nozzle inspection |
| Clogged or worn injector | Power loss, black smoke under load, large positive correction | Comparative return-flow test and bench test |
| Low compression on one cylinder | Hard starting, blue smoke, oil consumption, positive correction that remains after an injector swap | Compression and leak-down test, valve inspection |
| Valve timing or valvetrain | Mechanical noise, rough running, deviations on several cylinders | Timing check and valve clearance measurement |
Engine at operating temperature, stable idle, loads off (air conditioning, headlights, demisting) and no active faults that change fuelling. Otherwise the figures are not comparable.
Record each cylinder in the engine measuring block. Compare cylinders with each other and with the limits in the engine documentation, not with a universal forum threshold.
Take two or three readings a few minutes apart. A stable deviation on one cylinder is a clue; a value that jumps around points to another cause (air in the system, a sensor, compression).
Look for black deposits, tar or gas traces around the injectors: a sign of a blown sealing washer. Also check connectors and leak-off lines.
On common-rail systems with a return, measure the quantity returned by each injector under the manufacturer's conditions. An injector returning much more than the others is suspect.
Warning: Common-rail systems run at more than 1,000 bar. Never open high-pressure lines with the engine running and never put a hand near a leak: the jet can penetrate skin.
A cylinder with low compression receives a positive correction without the injector being at fault. A compression test separates the two before you buy parts.
Recondition or replace only the confirmed injector, with a new sealing washer. On many engines a new injector must be coded into the ECU with the code printed on its body (IMA, IQ or C3I), using a compatible scan tool.
After repair and coding, read the corrections again under the same conditions. Values should move closer together and idle should be stable.
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Create the diagnostic PDF reportImport codes from a photoA correction is an effect, not a cause. It does not say whether the problem is mechanical, hydraulic or electrical, and it does not say what the repair costs. Before replacing parts, check injector return flow, actual rail pressure, intake sealing and compression.
Do not clear faults before saving the capture, and do not run actuator tests on an engine with mechanical noise or suspicious pressure. If values change after fuel-system work, repeat the reading under the same conditions.