Preignition is an ignition that occurs before the spark from the plug—the uncontrolled firing of the fuel/air charge in advance of the normal spark ignition. It can be caused by a hot spot from a carbon or lead deposit, excessively hot exhaust valves, using high power when the mixture is too lean, or overheated or wrong heat range spark plugs that become heated to an incandescent or glowing state. It causes peak pressures in the cylinder at the wrong point in the cycle. Preignition can occur on one cylinder only, whereas detonation will normally occur in all cylinders.