Cylinder #9 Above Knock Threshold

Possible causes

• CommonFuel of a lower octane than the engine requires
• CommonCarbon build-up on the piston crown and in the combustion chamber, raising compression and holding heat
• CommonEngine running hotter than it should - low coolant, a failing thermostat or a fan not running
• CommonLean mixture on that cylinder from a blocked injector or a vacuum leak
• PossibleSpark plug of the wrong heat range, or one long overdue for replacement
• PossibleEGR not flowing, so combustion temperatures climb
• PossibleOverboost on a turbocharged or supercharged engine
• PossibleLoose or failing knock sensor reporting knock that is not happening
• RareMechanical noise from elsewhere in the engine being picked up as knock

What you might notice

• Metallic pinging or rattling under load, most obvious accelerating uphill or in too high a gear
• Loss of power, because the module is pulling ignition timing to protect the engine
• Poor fuel economy
• Check engine light on
• Often nothing noticeable at all - the protection works, and the code is the only sign

What to check, in order

1. Fill the tank with the octane the manufacturer specifies before doing anything else. If the code stops coming back, that was the answer and it cost one tank of fuel.
2. Check the coolant level and that the cooling fans actually run. An engine running hot detonates, and this code can be the first sign of a cooling fault.
3. Check whether the knock is reported on one cylinder or several. One cylinder points at that cylinder - its injector, its plug, its deposits. Several point at something shared: fuel, timing, cooling or boost.
4. Read the fuel trims and look for a lean bias, which raises combustion temperature and brings on knock.
5. On a high-mileage direct injection engine, consider carbon deposits. They build up on the piston crown, raise the effective compression ratio and cause knock on fuel that used to be fine.
6. Suspect the knock sensor itself last, and check its mounting torque before condemning it. A loose sensor reports knock that is not happening, and the module then pulls timing and power for no reason at all.