Engine feedback control system

An internal combustion engine control system for maintaining a desired fuel-air ratio comprised of a combustion chamber, an air induction system for delivering at least an air charge to said combustion chamber, a charge-forming system for supplying at least a fuel charge to said combustion chamber, an exhaust system for discharging combustion products from said combustion chamber, it combustion condition sensor for sensing the fuel-air ratio in said combustion chamber, a feedback control for receiving the output of said combustion condition sensor and adjusting at least one of said charge-forming and induction systems to maintain the desired fuel-air ratio, and testing means for comparing the output of said combustion condition sensor periodically under all engine running conditions to determine if said combustion condition sensor is providing an accurate signal, said testing means employing one method of testing during one type of running phase of said engine and another method of testing during another type of running phase of the engine.