P219A: Bank 1 Air-Fuel Ratio Imbalance
What to do
This is worth acting on even though the car may feel completely normal. It means one cylinder is getting a different mixture from the others — and the engine hides that by adjusting them all together, so nothing feels wrong until it becomes a misfire.
Ask about the injectors, not just the sensors. A single injector delivering slightly wrong is the classic cause, and catching it now is far cheaper than the catalytic converter damage a developed misfire can cause.
Mention any recent fuel that seemed poor, or a long period standing unused, both of which leave deposits that affect injectors unevenly.
Say whether the engine is at all rough at idle, even slightly. Idle is where an imbalance shows first, because each cylinder's contribution matters most when there is least of it.
What this code means
The cylinders on bank 1 are not receiving the same air-fuel ratio as each other.
This is one of the more sophisticated diagnostics on a modern engine, and understanding how it works explains why it catches things nothing else does. A wideband oxygen sensor is fast enough to resolve individual exhaust pulses. Each cylinder's exhaust passes the sensor in firing order, so within a single engine cycle the module can see a signature for each cylinder in turn — and compare them.
Cylinders that should be identical are not.
The reason this matters is that fuel trim hides it completely. Trim is applied to the whole bank, so if one cylinder runs rich the module leans out all of them to compensate. The average comes out right, the trims look healthy, and one cylinder is rich while the rest are lean. Nothing else on the vehicle would report that.
So this code catches a failing injector long before it misfires, which makes it a genuine early warning rather than a fault report. The cylinder still fires — it simply fires with the wrong mixture.
A bank-level code rather than a cylinder-level one means the imbalance was detected but not localised, which happens when several cylinders are affected or the signature is not clean enough to attribute. Look for causes that affect a bank rather than a cylinder: an intake leak feeding the manifold unevenly, a failing airflow sensor, or fuel pressure that the whole bank shares.
Likely causes
Start here
- Worn or leaking fuel injectors · specialist
- Vacuum leak in intake manifold or hoses · moderate
- Blocked or restricted injector nozzle · specialist
Less common
- Faulty mass air flow sensor · easy
- Low fuel pressure · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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