Today's cars have more computers than the Apollo lunar lander. Computer diagnostics are used to check engine performance as well as review various systems in a car.
Computers generate signals called data streams, which are used by constantly fine tune the engine. Diagnostic computers are interfaced (connected) with the car's computer to read and interpret the data streams flowing through the system. Typically, a problem with a computer-controlled car is indicated by a lit check-engine light on the dashboard, which could mean a bad sensor, malfunctioning electrical or mechanical component, or damaged wiring and plugs. When the computer recognizes a problem (or any deviation from normal) a "trouble code" is saved to the onboard computer's memory for later retrieval by the diagnostics system. The shop accesses these trouble codes which indicate the particular problem area to be checked out and repaired/adjusted.
Sometimes, though, there can be a performance issue, but the check-engine light is off, and there is no trouble code saved on the car's computer.
Effective computer diagnostics and repair demands state-of-the-art equipment, the latest information, and up-to-date training. Necessary equipment for the diagnostician includes:
A trained diagnostician must keep ahead of rapidly-changing technology, and a typical student from a technical college can take two years to get "up to speed" once hired.
Today's diagnosticians have access to over a million pages of technical information on fixing computer-related problems, HVAC (Heating Ventilation and Air Conditioning), Traction Control, Fuel Delivery Systems, and Performance Systems are all computer-controlled. Vendors of various car components, as well as the car manufacturers themselves, are constantly issuing TSBs (Technical Service Bulletins) to alert service professionals to new problems and repair strategies.
Typically, the diagnosis may start with a basic check of the computer's memory for any stored codes, in which case the simple replacement of a sensor may be all that's needed. If there are no codes, the technician might perform a "flight test," hooking a running car's performance system up to a handheld computer to monitoring the data stream for anything unusual. Then a "sensor stimulator test" would stimulate the sensors to simulate the car in various operating conditions. Failing that, a check of a particular component's electrical value with a digital volt-ohm meter might indicate it is not operating to the manufacturer's specifications.