The regulatory goal of OBD-I was to develop a diagnostics system which focuses on the emission control systems that remain effective for the vehicle's lifespan.
The basic function of OBDI systems was to monitor the oxygen sensor, EGR system, fuel delivery system and engine control module and provide an optical warning signal in the event of a malfunction. The Diagnostic Trouble Codes (DTCs) could be read via a flashing code without the aid of a testing device. Usually the warning signal was an illuminated LED on the ECM, the equivalent of a Malfunction Indicator Lamp (MIL), more commonly known today as the check engine light.
Each manufacturer used their own Diagnostic Link Connector (DLC), DLC location, DTC definitions, and procedure to read the DTC's from the vehicle. By connecting certain pins of the diagnostic connector, the 'Check Engine' light would blink out a two-digit number that corresponds to a specific error condition and technicians would have to count the number of flashes of the MIL to decipher the code. Another drawback was that the DTC's of some OBD-I cars were interpreted in different ways. Besides, some OBDI systems could only be accessed unless using dealer specific scan tools.