Why does it cost so much to diagnose a check engine light?

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We have all had it happen to us. We’re driving down the road, singing our heart out to the song on the radio, when the little amber icon on the dash suddenly appears. You scratch your head, thinking “that light looks familiar. I know I’ve seen it somewhere before.” So you take it to your local mechanic and tell him the sad story and ask him to let you know what the problem is. He informs you that there is a diagnostic charge (most shops will tell you around $100 plus or minus $10-20) to pin-point your issue. To which the most common reply is, “Why do you charge so much to plug in your computer and tell me what it is?” Well, I am glad you asked.

I am not sure when or where the idea arose that when your check engine light comes on that all you need to do is plug in a scanner and you’ll have your answer, but unfortunately it is not that simple. I imagine that the free code scans, or “free check engine light diagnostics” as the parts stores like to call it, is where it started. The parts guy comes out to your car as if they have this magical device that will let you know everything that’s wrong with your car, ready to sell you whatever is on the top of the list of possible problems with your car. “Well Sir/Ma’am, looks like you have a misfire code. These spark plugs and wires should fix it!” the man at the counter exclaimed. My favorite is when they sell you a gas cap with the confidence that it’ll fix it, after only reading that you have an evaporative code. What they fail to educate their customer on is that these are merely possible cures to their issues and not guaranteed solutions. Remember, their job is to sell you parts. An automotive technician’s job is to fix your car.

“How to know if you have a good mechanic.”

Now, before I go into what it actually takes to tell you why your check engine light is on, I will tell you a little more about what a DTC (Diagnostic Trouble Code) is. First of all, your check engine light will always come on for some reason or another. “But I’ve replaced everything in that system and the light is still on,” you tell me. Well you replaced everything except the part that is bad or you did something wrong along the way. There are a number of sensors that monitor your vehicle’s emissions system. Well, sensors monitor just about everything on your car, but we are talking about your check engine light which is strictly for your emissions system. Some of these sensors tell your Engine Control Module (ECM) if a different air to fuel ratio is needed for the current driving condition or if a certain system isn’t performing within the required specifications. Sometimes there is one sensor monitoring an entire circuit that consists of many different components and other times there are a number of sensors monitoring each part of a circuit. The more sensors on a circuit means the more accurate the code will be to narrowing down your issue. But that also means more parts that can break or fail costing you more in potential repairs.

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.