P2340: Cylinder 5 Above Knock/Combustion Vibration Sensor Threshold
What to do
Avoid heavy loads and towing, and ease off if you hear a rattle. On an engine this size, ask for the cooling system to be checked as well as the fuel and injectors — coolant picks up heat as it travels through the block, so cylinders further along run hotter and are the first to knock when cooling is not quite right. Note it if the fault appears in hot weather, in traffic or when towing, since that points the same way. The temperature gauge reading normal does not rule this out.
What this code means
Cylinder 5 is requiring more knock protection than normal. The mechanism is at P2336.
A fifth cylinder means a larger engine, and on larger engines cooling is no longer uniform. Coolant enters the block at one end and leaves at the other, picking up heat as it goes, so the cylinders furthest along the flow path run measurably hotter than the first ones. Engine designers compensate for this — but the compensation assumes the cooling system is working as designed.
When it is not, the cylinders that were already the hottest are the ones that cross the knock threshold first. So a knock code on a rear or middle cylinder of a straight-six or a V engine is worth treating as a possible cooling question rather than only a fuel or injector one, particularly if it appears in hot weather, in traffic, or when towing.
The things to look at are ordinary and mostly cheap: coolant level and condition, a thermostat opening late, a radiator partly blocked externally by debris or internally by scale, a water pump whose impeller has eroded, and air trapped in the system after a previous repair.
The temperature gauge is unhelpful here for the reason given at P2181 — it is damped enough to read normal across a wide band — so its sitting in the middle is not evidence that cooling is adequate.
Likely causes
Start here
- Low coolant level · easy
- Incorrect or low octane fuel · easy
Less common
- Blocked or restricted radiator · moderate
- Thermostat opening late · moderate
- Faulty water pump · specialist
- Carbon deposits in the combustion chamber · moderate
- Clogged fuel injectors · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
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
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P2340.