The OBD II Engine Management System (EMS) computer can be a single computer comprised of several solid-state components or a multi-microcomputer device. This computer controls the functions of the EMS and performs OBD II diagnostic routines. These two distinct portions of the OBD II EMS computer function in conjunction with each other. The computer architecture and software design allows the OBD II EMS computer to adapt its operating strategies to a variety of conditions to optimize the EMS. Federal guidelines require the EMS OBD II computer to continuously monitor the operating conditions of the EMS. It must also record and report any system or component failure that may cause tailpipe emissions to exceed typically 1-½ times the federal test procedure.
The OBD II EMS computer is specifically designed to perform powertrain system management and monitoring. Regardless of the manufacturers' design and implementation of operating strategies, all computers are designed and built following the same basic considerations.
All computers contain one or more microprocessors. They are constructed with a complex arrangement of digital circuits. One microprocessor may contain upwards of 250,000 logic circuits. These are housed in a silicon-based integrated circuit (IC) no bigger than an area of approximately 1/4 inch square. The microprocessor cannot perform calculations and decisions without instructions that are programmed into the computer.