Today’s vehicles are run by at least one, usually several computers or control modules. The engine is managed by an electronic control module, or ECM. These control modules monitor and control the fuel, emissions, temperature, the timing of the engine, braking, starting, charging, transmission shifting and speed. The electrical accessory systems that add luxuries like power windows, door locks, seats, mirrors, heating and air conditioning, are also run by a control module, called a body control module or “BCM”.
When one of these systems in your vehicle starts malfunctioning, a driver is alerted by either the illumination of a check engine light, or possibly, your vehicle won’t start, or the fuel economy and performance level has changed, or possibly another symptom. In order to properly diagnose what is going on with the vehicle and conclude what the proper repair would be, it takes knowledge, equipment and time for the automotive technician to run the needed tests. Unfortunately, when you tap into the vehicle’s ECM it does not tell you what is wrong. In fact, the ECM makes adjustments and changes to compensate for the symptom that it is seeing, so the vehicle will still operate as close to normal as possible. The diagnostic process starts with retrieving the code or codes from the ECM, These codes give you the circuit or circuits that are affected, and that the vehicle’s computer is seeing. From there a qualified diagnostic technician follows a diagnostic path of pinpoint testing to find the source of the problem.
A doctor charges to examine and run tests on patients that are having symptoms to find the answer. An automotive repair shop too, needs to charge for that testing time. To be honest this diagnostic process can be the most complicated part of vehicle repair. It takes a specialized diagnostic technician to perform the tests on the vehicle. These technicians, constantly continue to further their education, training and upgrading of their tools and equipment, since the systems on vehicles are constantly evolving and changing.
So don’t be surprised when you are quoted a diagnostic fee next time your vehicle acts up. In the end it really is less expensive to pay for a proper diagnosis, then it is to replace parts based on the hunch factor.