Abstract: A fail safe system for a cylinder pressure sensor is used for a spark ignition timing control system which calculates a physical quantity for combustion energy within the combustion chamber of an engine cylinder of an internal combustion engine and corrects the spark ignition timing on the basis of the physical quantity. The fail safe system sets a running average of a statistical dispersion of the physical quantity to a predetermined greater value when a specific driving condition, i.e. a high engine load and high engine speed condition is initially determined, and thereafter compares the running average with a predetermined value when the specific driving condition is detected again to detect abnormality of the cylinder pressure sensor.