Abstract: A system is provided for controlling the ratio of fuel to air in an internal combustion engine to protect the catalyst substrate of a catalytic convertor from thermal damage. The system comprises detecting a misfire rate and adjusting the amount of fuel within the system accordingly. More particularly, the detected misfire rate is compared to at least one predetermined misfire rate within the engine control unit. If the detected misfire rate is not equal to the predetermined rate, the ratio of fuel to air is changed a calibrated amount. Accordingly, when the misfire rate exceeds a predetermined rate, additional fuel is added to the system. By increasing the ratio of fuel to air, the oxidation reaction is reduced and the exothermic activity within the catalytic converter is lowered.