When we refer to diagnostics, we're generally speaking of troubleshooting an electrical component in the vehicle such as a sensor failure for the engine management system that would turn on the "Check Engine" light. But realistically a diagnostic could be anything where a specialized piece of equipment is needed to pinpoint test a component in any system in your vehicle.
There are a number of reasons why diagnostics is a separate charge. Aside from the high cost of the highly specialized testing equipment and the extra training and experience needed by technicians to be able to perform that kind of work, the biggest cost factor when speaking of diagnostics is time. The complexity of today’s cars has really fueled the need for specialized equipment, vast information systems and extensive training to diagnose and properly repair electronic problems. These electronics are not just in the engine controls these days; computers control the instrument gauges, brake operations, power steering, radios, power windows, door locks and many other items that we may not realize are computer controlled. When a customer takes a car in for a performance repair, he or she is essentially asking the technician to "analyze my computer system."
For example: When diagnosing a "Check Engine" light problem, it generally takes between an hour and a half and two hours for a technician to pull the trouble codes from the engine computer and perform the associated test procedures for the components relating to the code.
Vehicles today are made up of multiple microprocessors that interlink all the systems under the hood and throughout the vehicle. Not only do today's cars contain more computer power than the Apollo spacecraft that landed on the moon but the average luxury automobile has more than 100 million lines of code, spread across all of its microcontrol units which is twice the amount of code found in a desktop operating system.
Good diagnostic equipment can run over $9,000 and cost $1000 a year to keep updated. Add to that information systems, special tools and training and you can see why the cost for a shop to do an accurate diagnosis can be high.
While many people think that repair shops have a piece of equipment that plugs into the vehicle and simply tells you what's wrong, that just isn't the case. The most any piece of diagnostic equipment can do is read a trouble code from a system control module and display it with a brief description of what the code refers to along with live sensor data so that the service technician can see what the control module sees from the individual components within the system. From that point the service technician will look up the diagnostic procedure in our automotive database and perform individual tests on the related components as per the diagnostic flow charts.
When your check engine light comes on, here is the process that a technician has to go through to pin point your problem:
First, they will scan for DTC’s and verify any drivability complaints.
Using his or her knowledge and years of experience, they will check for obvious faults that could lead to your codes such as loose or broken wires, rotted vacuum lines, or unplugged sensors or electrical components.
They may need to monitor PIDs (Parameter ID’s) on their scan tool and see what all the sensors in the faulty circuit are doing and how they are talking to each other.
Now, the often time consuming part, running tests. After gathering all of the preliminary data, the technician will pull up wiring diagrams and check tech forums (both of these come from costly monthly subscription services that most shops/technicians pay for) and come up with a logical flow chart for testing the circuit. Sometimes these test procedures are very quick. Some are extremely time consuming, some times to the point that your service advisor may call and request more money for more diagnostic time.
At the end of the testing, the technician will have found a single (sometimes multiple) failed part(s) and will advise you to replace those things and then recheck the system. That’s right, recheck the system. Sometimes, the first failed parts found are merely a starting point.
The point is, getting your check engine light codes is a single step in a very complex and time consuming procedure to find out why the light is on in the first place. To get the right answer for your problem it involves using expensive equipment and knowledge gained from schooling, classes, or years spent fixing cars. Not to mention the overhead of the shop you brought your car to. You pay your doctor without a second thought when you go into his office with a cold, for him to just say, “Yup, you have a cold. Get rest.” Automotive technicians are arguably the same as physicians, but for your car. I wish that there was a single device that could tell you everything wrong with your car. I imagine that someday, likely in the not too distant future, cars will be smart enough to pinpoint exact failures, but for now, you will have to rely on the knowledge and experience of automotive diagnosticians.