P21A0: Cylinder 5 Air-Fuel Ratio Imbalance
What to do
Ask whether this cylinder is rich or lean relative to its neighbours. That single fact from live data halves the diagnosis, and it is quick to read.
Ask what the labour is to reach this injector before agreeing to anything. On a six or eight cylinder engine, access dominates the bill — and if the injector is coming out anyway, it is worth having the others tested at the same time.
This is not a misfire. The cylinder is firing with the wrong mixture, which is the earlier and cheaper stage of the same problem.
Mention any rough idle, and any recent poor fuel or long period standing.
What this code means
Cylinder 5 is receiving a different air-fuel ratio from the other cylinders.
A cylinder 5 code means a five, six or eight cylinder engine, and on anything with two banks it is usually the first cylinder of the second bank — which is worth establishing before diagnosing, because it changes what "the other cylinders" means.
The comparison is made within a bank, not across the engine. Each bank has its own wideband sensor, so this cylinder is being compared with its bank-mates rather than with all of them. A whole bank running differently would not show as an imbalance at all — it shows as a fuel trim difference between banks, which is a separate code entirely.
That distinction is genuinely useful: an imbalance code means one cylinder differs from its neighbours, which rules out anything affecting the bank as a whole.
The causes are the family's, local to this cylinder: the injector, its intake runner, its compression, or its seal.
On a larger engine, ask what the labour is to reach this injector before deciding. Access dominates the cost on a V6 or V8, and if an injector is coming out it is worth knowing what the others' flow rates look like — imbalance is a comparison, so a set that is drifting together can eventually mask the very fault this code exists to find.
Likely causes
Start here
- Worn or leaking fuel injectors · specialist
- Blocked or restricted injector nozzle · specialist
- Leaking intake manifold gasket · moderate
Less common
- Low engine compression · specialist
- Vacuum leak in intake manifold or hoses · moderate
Symptoms
Components involved
Information, not instruction
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Community experience
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