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Toyota introduced a new engine control system using a new microprocessor during the Fall of 1982. The new control system is used effectively for more complex application to engine and automatic transmission control. It controls air-fuel ratios in combination with the transmission shift control to achieve good fuel economy, driveability, as well as emission reduction. This system includes a self diagnostic capability, in which the electronic control unit (ECU) diagnoses system abnormalities, stores them in the memory and turn on the CHECK ENGINE lamp. To assure the proper system operation against any failure of the processor, the electronic control unit has a back up circuit which executes the predeterminded operation of fuel injection and spark timing. For this system, a new 12-bit microprocessor capable of high speed real time processing was developed.