P21A1: Cylinder 6 Air-Fuel Ratio Imbalance
What to do
Ask whether this cylinder is rich or lean compared with the others — that comes from live data and immediately narrows what needs checking.
Take it seriously despite the car feeling normal. This kind of fault is invisible in the usual fuel readings, which is exactly why the code exists — and it is the early stage of what eventually becomes a misfire and a damaged catalytic converter.
Ask whether fuel rail pressure has been checked, particularly on a six-cylinder engine where this cylinder sits at the end of the rail and feels a shortfall first.
Mention any rough idle, poor fuel recently, or a long period standing unused.
What this code means
Cylinder 6 is receiving a different air-fuel ratio from the other cylinders.
This completes the cylinder-level set in this batch, and it is a reasonable place to record what the imbalance family is worth as a whole.
These codes exist because fuel trim is a bank-wide average and averages hide the thing that matters. A single injector delivering ten per cent extra is invisible to trim — the module leans the whole bank, the numbers look healthy, and one cylinder runs rich while its neighbours run lean. The engine feels fine. The catalytic converter does not.
So the family's value is timing. It reports a fault at the stage where a cylinder still fires reliably, which is one or two years before it becomes a misfire code and a converter replacement.
Reading the codes together tells you the scope:
- One cylinder — that injector, runner, seal or compression.
- Several on one bank — that bank's fuel supply or intake.
- A bank-level code with no cylinder named — the module saw the imbalance but could not attribute it, usually because more than one cylinder is involved.
Where an imbalance code and a misfire code name the same cylinder, they are one fault at two stages — and the misfire is the one to act on.
On a straight six, cylinder 6 is the furthest from the fuel supply, so a rail marginally short of pressure shows here first.
Likely causes
Start here
- Worn or leaking fuel injectors · specialist
- Blocked or restricted injector nozzle · specialist
- Low fuel pressure · moderate
Less common
- Leaking intake manifold gasket · moderate
- Low engine compression · specialist
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.
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