What comes to mind when you hear the words, “your vehicle needs a computer diagnosis to find the problem?” Do you envision a technician connecting the car to a big machine with flashing lights that “beeps” a few times then prints out a description of the problem and its solution? Unfortunately, it’s not that simple.
Modern electronic engine control systems “know” and monitor the operating parameters of every component. When the
input from a sensor falls outside normal limits for too long, or the output signal to an actuator repeatedly fails to generate the expected result, the PCM stores a Diagnostic Trouble Code (DTC).
If a stored code indicates a problem that may increase exhaust emissions, the PCM also illuminates the “Check Engine” light on the dash. However, many types of codes can be set with no indication to the driver.
To access DTCs, technicians connect a “scan” tool to a Diagnostic Link Connector that is commonly located under the driver’s side of the instrument panel. The scan tool displays any stored codes, and many motorists think that’s where diagnosis ends. They question why they should have to pay, sometimes dearly, for such a simple procedure.
In reality, pulling trouble codes is just the first step in a computer diagnostic procedure. DTCs don’t tell you if a part is bad, they only indicate that the computer has seen something it didn’t expect in a particular circuit. The problem may be the part associated with the trouble code, but it could just as easily be a shorted or open circuit in the part’s electrical wiring.
Sometimes, trouble codes are set when there is nothing wrong with the engine control system. This happens when a mechanical problem, like a vacuum leak, creates engine operating conditions that cause a sensor to generate an out-of-spec signal. The PCM will then attempt to compensate, which may cause an actuator to operate outside its normal range, setting yet another trouble code!
To help pinpoint the problem, a technician must perform additional tests. These can range from mechanical checks, like engine compression, to more in-depth electronic diagnosis. One common operation involves using test equipment to access the engine control system network data stream. This allows the technician to view real-time sensor data and manually operate the system actuators.
The ability to determine which additional tests are needed, and to accurately interpret both test results and computer network data, comes from training and experience. Today’s technicians use vehicle computer diagnosis in much the same way surgeons employ medical testing. In both cases, combining test results with expert knowledge and skilled hands can lead to a diagnosis and a cure.