What is a “Diagnostic”

By Brian Klaubert

Just what is involved with the diagnostic charge when getting your car repaired? Many people are confused about this.

Many people believe that the car is plugged into a computer and that the computer prints out everything that is wrong with the vehicle.  This is not correct. A thorough diagnostic procedure involves inputs from several different sources.  The computer is only one tool, and not the final judge.

On 1996 and later vehicles the vehicle’s own onboard computer provides a significant amount of data, but not answers. On 1995 and earlier vehicles there is very limited data available through the computer.

Generally, the first step is to connect a diagnostic scanner to the vehicle and look for any malfunction codes that are present. The technician then looks at raw data streaming through the vehicle’s computer, if available. Frequently the malfunction code present indicates the symptom, and is not the root problem. The technician’s training, experience, and inherent trouble shooting skills now become very important.

Frequently the next step involves driving the vehicle. With some preliminary information, the technician is now prepared to pay particular attention for specific symptoms, sounds, and characteristics related to the customer’s concern.

No car is perfect. Each vehicle model and year has characteristic problems. The car manufacturer’s publish Technical Service Bulletins describing common problems and their symptoms. A good technician will check the bulletins for your car to see if the problem is similar to one of those described. The better diagnostic computers also have detailed troubleshooting information and common faults that the manufacturer’s do not report. This information is updated periodically, either quarterly or at least annually.

The next step is to follow diagnostic trouble shooting diagrams. These diagrams indicate tests to be performed, and based upon the result, additional tests or conclusions. Some diagnostic procedures are very short. Others go on for several pages. The technician’s knowledge and experience guide him on where to begin in the troubleshooting procedure.

The last step is to test the suspected bad part isolated from the rest of the system. This is not always possible, but is desirable if practical.

With all of the information that is available, the knowledge, experience and skill of the technician becomes more important to sort through what is pertinent to the current problem, and what is extraneous. Ask about the experience and certifications of the technician who will work on your vehicle. Remember that the person you speak with in the customer area is not the person who will be diagnosing your vehicle.

Performing the diagnostic evaluation takes time, several different computers, subscriptions and updates to several different databases, and a knowledgeable and experienced technician. That is why there is a charge.

Brian Klaubert owns Christian Brothers Automotive – Hamilton Mill, in Dacula.