Engine control system and method

An engine control system for an internal combustion engine having a combustion chamber, an air fuel induction system for delivering an air fuel charge to said combustion chamber, an ignition system for igniting the charge in said combustion chamber, an operator demand throttle control for controlling the engine output in response to operator demand, a throttle position sensor for sensing the position of said throttle control, control means for controlling said air fuel charging system and said ignition system in response to sensed engine condition including the condition of said throttle position sensor for controlling engine operation, means for detecting whether the output of said throttle position sensor is accurate by checking to determine if there are changes in its output signal when the engine is judged to be operating in a speed range where throttle position changes are not normally encountered, and providing an indication of abnormality of the throttle position sensor output if the output varies within that speed range.