What it means
The ECU continuously trims individual cylinders to equalise air-fuel ratio across the bank using ion-current or per-cylinder lambda feedback. For cylinder 9 the per-cylinder fuel correction has hit its software-side maximum or minimum without restoring balance, indicating the underlying mechanical or fuelling deviation is too large to compensate.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine control module uses per-cylinder feedback to adjust fueling and balance combustion across the engine. This code means the correction for cylinder 9 has reached its adjustment limit without resolving the air-fuel imbalance, pointing to a cylinder 9 fueling or mechanical problem such as injector flow drift or low compression.
Common causes
Most likely first.
- Fuel injector for cylinder 9 has drifted in flow rate
- Reduced compression on cylinder 9
Symptoms
- Check-engine light
- Rough idle
- Engine hesitation or reduced power
- Increased fuel consumption
How to diagnose it
- Record freeze-frame data and review any per-cylinder balance or fuel-correction data to confirm that cylinder 9 is the one reaching its limit.
- Inspect cylinder 9's injector connector and wiring for looseness, damage, or corrosion.
- Check for intake or vacuum leaks near cylinder 9 that could disturb its mixture, using the vehicle's layout to identify the relevant area.
- Test cylinder 9's injector operation and flow against the manufacturer's procedure and compare it with other cylinders.
- Perform a compression test on cylinder 9 and compare the result with the manufacturer's specification and the other cylinders.
Common fixes
- Repair a loose or damaged cylinder 9 injector connection.
- Correct an intake leak affecting cylinder 9.
- Clean or replace the cylinder 9 injector if testing confirms abnormal flow.
- Repair the mechanical cause if cylinder 9 compression is low.
Can I keep driving?
Short, gentle trips may be possible if the engine runs smoothly, but avoid heavy loads and arrange diagnosis soon. Stop if the check-engine light flashes or the engine runs very roughly, as a persistent imbalance can raise emissions and damage the catalytic converter.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.