(September 17, 2012) ORLANDO, Fla. — When your auto repair shop recommends a "computer diagnosis" it sounds serious, and everyone knows that serious vehicle problems can lead to serious repair bills. However, AAA experts advise consumers not to be overly concerned because computer diagnosis is a common practice for identifying problems on today's microprocessor-controlled cars.

"If your vehicle needs a computer diagnosis, don't be alarmed. This is often the most efficient and cost effective way for an automotive technician to diagnose a problem," says John Nielsen, director, AAA Automotive Engineering and Repair. "Just be sure to have the repair facility explain what will be done before you authorize the work because 'computer diagnosis' is a generic term that can cover a wide range of operations."

To help motorists better understand computer diagnosis, AAA Automotive Engineering offers the following information. Based at the association's national office in Heathrow, FL, the automotive engineering team's goal is to provide motorists with unbiased information and advice on automotive technology.

   • Modern vehicle electronic control systems "know" and monitor the operating parameters of every component. When the Powertrain Control Module (PCM) that manages the system sees a signal that is outside normal limits, or fails to see an expected change in a signal, it stores a Diagnostic Trouble Code (DTC).

    • 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, but that is only the beginning of a full computer diagnosis.

    • DTCs don't tell a technician if a particular part is bad, they only indicate that the PCM has seen something it didn't expect in a certain circuit. The problem might be the part, but it could just as easily be an issue with the circuit's electrical wiring.

    • Sometimes, DTCs are set when there is nothing wrong with the electronic control system. This happens when a mechanical problem, like an engine vacuum leak, creates operating conditions that cause system components to generate signals outside their normal range.

    • To pinpoint a problem, the technician starts with the DTC, then performs additional tests. These can range from mechanical checks, like engine compression, to in-depth electronic diagnosis. One common procedure uses special test equipment to access the electronic control system data network and monitor real-time signals from the system components.

The ability of technicians to determine what additional tests are needed, and to accurately interpret both test results and computer network data, comes from extensive 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 an accurate diagnosis and an ultimate cure.