P2B18: Cylinder 5 Injection Pulse Offset Not Learned

What it means

The ECM has not yet learned the per-cylinder injection pulse offset for cylinder 5. Combustion equalisation between cylinders cannot be applied until adaptation completes.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The ECM stores a learned injection pulse offset for each cylinder so it can balance combustion. P2B18 means it has not completed that learning for cylinder 5, so cylinder 5's individual correction is unavailable; this can happen after an ECM reset or injector replacement.

Common causes

Most likely first.

1. Adaptation lost after battery disconnect or ECM reset
2. Injector replaced without learn procedure

Symptoms

• Check-engine light
• Rough idle
• Slightly reduced engine performance

How to diagnose it

1. Read freeze-frame data and confirm P2B18 is current, noting whether it appeared after a battery disconnect, ECM reset, or cylinder 5 injector work.
2. Check for other cylinder-specific or fuel-system codes that could interfere with learning before focusing on the adaptation.
3. Inspect the cylinder 5 injector connector and nearby wiring for loose connections, damage, corrosion, or signs of recent work.
4. Use a scan tool that supports the vehicle's injector adaptation to check whether the cylinder 5 offset is unlearned and whether the learn procedure is available.
5. Perform the manufacturer's cylinder 5 injector learning procedure under its stated conditions, then rescan to confirm the offset has been stored.

Common fixes

• Complete the manufacturer's cylinder 5 injection-offset learning procedure.
• Repair a loose, damaged, or corroded cylinder 5 injector connector or wiring.
• If learning will not complete, test the cylinder 5 injector and related fuel system against the manufacturer's specification.
• Replace a confirmed faulty cylinder 5 injector, then perform the required learning procedure.

Can I keep driving?

If the engine only idles slightly rough and has no other warning signs, a short, gentle drive may be possible, but arrange diagnosis soon because cylinder 5 combustion balancing is unavailable and emissions may increase. Stop driving if the engine runs severely rough, loses substantial power, or the warning changes to indicate an urgent fault.