A car has at least one computer, but most have an engine control module (ECM), a body ride control unit, an ABS computer and a climate control computer. How many of these computers depend on the vehicle year, make and model and the options the vehicle has. If the vehicle does not have ABS brakes, it will not have an ABS computer. If the vehicle does not have climate control, it will not have a climate control computer.
The main computer is the engine control module. This may also be called the programmable control module. This computer takes its inputs and outputs through the various sensors on a vehicle. Sensors that give input give the computer information to send back to the engine via the output sensors.
Some of the sensors include a mass air flow sensor, a coolant temp sensor, cam sensor, crank sensor, idle air control motor, EGR valve, throttle position sensor and a knock sensor. Each sensor has its own job in telling the computer what the conditions are such as air quantity and quality, temperature of the coolant and where the throttle is. The computer uses this information to make infinite changes to the air-fuel ratio, turn the fans on or off and change the idle speed and injector timing, along with many other things needed to run the vehicle.
This part of the vehicle's running system is diagnosed by the engine control module. It lets you know when something is not working by turning the "check engine" or "service engine soon" light on. The ECM sends "codes" that turn the light on. A scanner is hooked up to the port under the driver's side dash so that the codes can be read. Because you cannot visually see if a sensor is bad, you must rely on the ECM to tell you this information. The scanner will tell you which sensor is out of range or not working at all.
The ECM cannot diagnose itself. If an ECM goes bad, the vehicle may run badly or not at all. If the vehicle is running badly, a sensor could be out of range or bad, or the computer could be bad. When you hook up the scanner, you may get codes that do not exist or you may get codes that exist but do not make sense. This is a good sign that the computer itself is not working properly and should be replaced.
When you are working with the computer, extreme care must be taken. A friction shock from your fingers can damage the computer. Ground yourself before touching the computer. Most of today's computers must be programmed for the specific vehicle (getting "flashed"). You will need the VIN and the mileage of the car in addition to the numbers on the outside of the computer. The staffers at the auto parts store or the dealer should know that the PROM must be removed from the old computer and put into the new computer prior to "flashing" the computer. If they neglect to move the PROM or they say they will "flash" the computer without having your old computer, be sure to remind them that they cannot flash it without the PROM.
