P2339: Cylinder 4 Above Knock/Combustion Vibration Sensor Threshold
What to do
Avoid heavy loads and ease off if you hear a rattle. Ask for that cylinder's injector to be checked — a partially blocked injector makes one cylinder run lean, and a lean mixture burns hotter, which is what causes the knock. It often will not show up as a fuel mixture fault, because the sensors measure the whole bank and one lean cylinder averages out, so this code can be the only sign of it.
What this code means
Cylinder 4 is requiring more knock protection than normal. The mechanism is at P2336.
The cause specific to one cylinder that people find least intuitive is a lean mixture, because lean sounds like less fuel and therefore less to go wrong. In fact a lean mixture burns hotter — there is excess air, so combustion is more complete and more of the heat goes into the chamber rather than into vaporising surplus fuel. Extra fuel is a coolant as well as a fuel, and taking it away raises combustion temperature, which is exactly what promotes knock.
On a single cylinder that almost always means its injector: partially blocked by deposits, or delivering a poor spray pattern. Both reduce that cylinder's fuelling while its neighbours are unaffected, and neither necessarily sets a fuel trim code, because the overall mixture measured at the oxygen sensor can still look correct while one cylinder runs lean and another compensates.
That is why a knock code can be the first and only sign of an injector problem — it catches a per-cylinder fault that whole-bank measurements average away.
An intake leak drawing unmetered air into one runner does the same thing by a different route and is worth checking on the same visit, particularly on an engine where the inlet manifold gasket is a known weak point.
Likely causes
Start here
- Clogged fuel injectors · moderate
- Vacuum leak in intake manifold or hoses · moderate
- Incorrect or low octane fuel · easy
Less common
- Carbon deposits in the combustion chamber · moderate
- Leaking intake manifold gasket · moderate
- Weak fuel pump · 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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