The Micro Computer Unit, or MCU, is the heart of the electronic control system. The MCU receives signals from various engine sensors to constantly monitor the engine operating conditions, then it uses this information to make adjustments in order to achieve the optimum performance and economy with a minimum of engine emissions. The MCU monitors the oxygen sensor voltage and, based upon the mode of operation, generates an output control signal for the carburetor stepper motor or mixture control solenoid. If the system is in the closed loop mode of operation, the air/fuel mixture will vary according to the oxygen content in the exhaust gas and engine operating conditions. If the system is in the open loop mode of operation, the air/fuel mixture will be based on a predetermined ratio that is dependent on engine rpm.