P219F: Cylinder 4 Air-Fuel Ratio Imbalance
What to do
Ask whether this cylinder is running rich or lean compared with the others. The garage can usually see this in live data, and it halves the diagnosis — rich points at the injector delivering too much, lean points at an air leak or a blocked injector.
This is not a misfire, and that is the good news. The cylinder is still firing; it just has the wrong mixture. Catching it now is far cheaper than waiting for the misfire that follows, which can damage the catalytic converter.
Mention any recent poor fuel, or a long period with the car standing unused. Both leave deposits that affect one injector more than the others.
Say whether the engine is at all rough at idle, even slightly — that is where an imbalance shows first.
What this code means
Cylinder 4 is receiving a different air-fuel ratio from the other cylinders.
A cylinder-level imbalance code is more valuable than a bank-level one because it has done the hardest part of the diagnosis already. The module has resolved the individual exhaust pulses in the wideband sensor signal and attributed the imbalance to one cylinder.
Which turns a broad investigation into a short list, all of it local to that cylinder:
- The injector. Delivering more or less than its neighbours — worn, partly blocked, or with a failing seal. This is the leading cause.
- The intake runner. A leaking gasket or a cracked runner leans this cylinder alone.
- Compression. A cylinder that seals less well takes in less air, so its ratio differs even with a perfect injector.
- A leaking injector seal at the head, which lets combustion pressure escape and disturbs the charge.
The direction matters and is worth asking for. Live data usually shows whether this cylinder is rich or lean relative to the others — rich points at an injector over-delivering, lean points at a leak or a restricted injector. That single fact halves the list.
Note that this is not a misfire code and should not be treated as one. The cylinder is firing. It is firing with the wrong mixture, which is a smaller fault caught earlier — and left alone, it becomes the misfire code.
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
- Leaking injector seal · specialist
Symptoms
Components involved
Information, not instruction
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Community experience
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