Understanding the Ten Step Auto Diagnostic Procedure

by | Jun 13, 2017 | Auto Questions, Car Problems & Repairs, Preventative Maintenance

How do we diagnose complex “issues” when they go wrong?

Often, when an appliance stops working, we don’t even think about repair; we simply purchase a new one — there is limited or no diagnosis involved. We may check to make sure everything is plugged in, that a circuit breaker hasn’t tripped, or even check the internet to see what we find, but that’s typically the extent of it.  Think about it. For example, many microwave ovens can be replaced for less than $100, while the average cost of a repair (according to Angie’s List) is about $133.  That’s because repairing one typically takes a technician up to an hour to diagnose and repair the problem. Plus on top of the cost for the technician’s time, there is the cost of the replacement parts. Combine this with the sheer inconvenience of the repair process like schedule coordination or shipping, and it becomes clear why this type of repair process only works for more expensive items.

When our gas fireplace stopped working, we called a company that specializes in fireplace service and repair. The process was interesting because it brought to light an interesting concept of what they consider “diagnosis.” For this particular company, “diagnosis” was simply “calling the manufacturer hotline and having them identify what should be replaced.”

This was done based on symptoms.  In our case, the identified symptom was “not switching on intermittently.” The answer supplied by the hotline was, “it’s the electronic module.” So, the $350 electronic module part was ordered. When it arrived, the technician came by to replace it, but the fireplace was working! He suggested that we call him when it fails again, “I will come by, check it, and replace the module,” he assured us.

Well, sure enough, it failed again, and they came by and installed the module, but it still didn’t work! The technician went back to the hotline again, and there was a new suggestion, “replace the auxiliary power supply.” When I suggested to the service department that this was not a diagnostic process but simply trial and error, she gave me an automotive repair analogy. “If you took your BMW to a dealer this is what they do. They call the manufacturers’ hotline, and they tell the mechanic what to repair.” I could not believe what I was hearing!!! This may be how automotive repair diagnosis is perceived, but it is certainly not how it’s done! Well, let me back up a bit, there might be a grain of truth to this regarding calling a hotline, but it’s certainly not industry practice to call a hotline and have them tell you what to replace.

But then, what is the “automotive diagnosis process? How do we diagnose and repair a complex vehicle with dozens of mechanical systems coupled to dozens of computer modules all hooked together on a network?

The ten point diagnostic procedure relies on good communication, particularly from the customer to the technician regarding what’s going wrong with the automobile. A skilled technician needs to have good “critical thinking” skills plus very good “problem solving” skills. These skills are essential because modern vehicles with dozens of computers can go wrong in an infinite number of ways.

Step One: The service writer interviews the customer and documents the symptoms on a diagnostic form.

Step Two: The technician confirms the problem, and starts with some basic checks. For example, check the battery and make sure the connections are in good condition.

Step Three: The technician then checks for any service updates related to the symptoms and system with the problem. This is where a layperson may get the idea that “If you took your BMW to a dealer this is what they do, they call the manufacturer and they say what to repair.” It’s true that there are “pattern failures.” Things can go wrong because of a faulty batch of parts. These faults include related recalls and service updates. Once the technician is sure there are not any quick fixes he is on his own!

Step Four: The skilled diagnostic technician, reviews the system being worked on and starts a logical step by step diagnostic procedure.

Step Five: At this stage, the mechanic might call a technical support hotline, and go over the process he is using. He might share data, and the support hot line might link with the technician to work together to diagnose the problem in the most efficient way.

Step Six: Once the problem has been diagnosed the technician reports his findings to the service writer.

Step Seven: The service writer makes an estimate of what it will cost to complete the repair and calls the customer to get approval to proceed. Sometimes we find a defective part, and we have to replace it to continue the diagnosis. This could be the only problem or the first step in the diagnosis.

Step Eight: Once the problem is repaired, the technician reassembles and tests the vehicle. He saves the data in the computer. Sometimes we perform a complete electronic scan of the car and save it in the computer.

Step Nine: The service writer then finishes documenting the repair.

Step Ten: Once everything has been documented, the vehicle is given a final test drive and check before it is parked and is ready to be returned to the customer.

Is this how you thought the auto repair diagnostic process worked?  I think this is certainly how I expected our gas fireplace repair process to proceed, but I was wrong about that.  Have you ever thought a diagnostic process would go one way and it went another?  Share it with us in the comment section.

Also, be sure to fill out a diagnostic sheet before your service visit if you are experience an automotive issue.


  1. james b carey

    can i train someplace near buffalo ny to be an expert strictly in check engine light repair issues? if so please provide a contact name,number, or location…thanks

    • Brian England

      Hi James, It’s not possible to train just for Check Engine Light, the codes, data behind the codes and the mechanical system involved have to be understood. This is why it takes a skilled technician to diagnose the data behind Check Engine Light. It’s commonly misunderstood that the code that can be easily accessed tells you the problem, this is just not the case and is why so many people waste their money on parts that don’t fix the problem.
      To become a skilled technician takes up to 10 years.


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About the Author

About Brian England

Brian England, the current president of BA Auto Care (formerly British American Auto Care), got his start with an auto apprenticeship in a small town in northwest of London. He came to the U.S. in 1972 to work for a Land Rover dealer in Rockville, MD. A few years later, he started British American Auto Care with his wife, Jennifer. Brian is a big believe in preventative maintenance, and his philosophy is to encourage and educate drivers on the benefits of a regular maintenance plan.