Contrary to what some people believe, onboard diagnostic systems were not created by the manufacturers to make it impossible for people to work on their own cars. Rather, they were developed in response to government regulations for tighter emissions controls. It all began with the requirement of catalytic converters in the exhaust. The cats clean up the exhaust gases by converting the carbon monoxide to carbon dioxide and breaking up the oxides of nitrogen, but they need to work within a certain air/fuel range, just like an engine. If the exhaust is too rich or too lean, the converters can overheat and fail. So to make the cats live, the manufacturers needed to abandon carburetors and distributors for electronically controlled fuel injections and distributorless ignitions.
In the early systems, the computers that controlled the electronic carburetors and throttle-body fuel-injection systems were pretty advanced for their time, though they would be considered stone-age technology today. They used input from oxygen sensors in the exhaust stream to determine the proper air/fuel ratio the engine should run at to produce the least possible emissions and were capable of warning the car owner, via the check engine light, if the engine was generating exhaust emissions outside the range of what the catalytic converters could handle. Of the three domestic manufacturers, GM was the early pioneer in electronic controls, and its system was the most comprehensive and best organized—it used the same data link connector and trouble codes across its model line. GM also provided a scan tool (the Tech 1) to its dealership technicians, used to read the codes and diagnose the faults.
That's not to say the system was without problems. The number of trouble codes was limited and the descriptions were rather vague, often leaving technicians to guess at what the real cause of a trouble code was. Things were even more difficult for Ford and Chrysler technicians. The locations and shapes of the data connectors varied from car line to car line, as did the procedures for retrieving the trouble codes. Of course, their codes were different from GM's codes. Imagine what havoc that played on guys at independent repair shops. They had to have myriad scanners, connectors, cables, and adapters, plus the expensive software for each manufacturer. You can see how difficult and expensive diagnostic repairs are.
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