How to use an automotive diagnostic tool

How to use an automotive diagnostic tool

In this article we will try to gather all the basic information about the automotive diagnostic tools, how they work, how to use them, and if they can detect mileage correction procedures that the car has undergone in the past.


In the recent past, tools for car diagnostics were prohibitively expensive. Before 1996, any auto technician had to get a tool compatible with only one car brand for thousands of dollars. Even the introduction of the OBD-II (which stands for On-Board Diagnostics) connector did not ease this burden on the wallet of many car owners. Surely, it made a revolution, but it didn’t happen overnight.

Today, you can buy a simple “code reader” for less than a can of gasoline, and some accessories can even turn your smartphone into a scanning tool. Since the bulk of the information you need to interpret from the resulting code can be found online, an engine “check” should no longer trigger a hasty trip to the auto shop.

Before buying an automotive diagnostic tool, it is important to understand that they are no magic pill. Just like the “check engine” light, these tools do not tell you how to fix the problem (however, they certainly can give you a clue). In some cases, it won't even tell you what the problem is. All it will do is show an error code, or several codes of reference.


When the “check engine” lights up on the dashboard, it means that there is some sensor somewhere in your engine, exhaust, or transmission, that has actually provided unexpected data to the cars ECU (Electronic control unit). This may indicate a problem with the engine’s system, a sensor failure, or sometimes even a wiring problem.


There was a time when аutomotive scanners were only available from specialized vendors. Today, this is not the case, and you can buy inexpensive scanning tools in retail or online. Also, consider renting one or more and testing it out before buying.


Turn off the car and remove the ignition key from the ignition lock. Carefully insert the code reader connector into the diagnostic connector. If it does not fit, then make sure that you are connecting the plug the right way (however it’s shape is rather foolproof), and that you have correctly identified the OBD-II connector.

If the diagnostic connector is inserted correctly, you can insert the ignition key and turn it to the ON position. This will provide power to the device for reading the code.

Note: You may need to enter the VIN number, engine type, or other information. At this point, the decoder will be ready to do its job. The basic device will simply provide you with all the basic codes, while other tools will give you the codes and additional data to work with.


With basic tools, you will have to write down the error codes and do some research. For example, if you find the code P0401, a search on the Internet will show that it indicates an error in one of the oxygen sensor heater circuits. It doesn't tell you exactly what's wrong, but it's the part of the engine where you should start looking for the problem.

Advanced scanning tools could tell you exactly what the error code means. In some cases, it will even provide you with a step-by-step troubleshooting procedure.

The next step is to determine why the error code appeared. The easiest way to do this is to find potential causes and eliminate each one by one.

In our example with the error code P0401, further investigation will show that this error indicates a defect in the oxygen sensor heater circuit. This may be caused by a faulty heating element or a wiring problem.

In this case, we will need to check the heating element first. Maybe the heating element is short-circuited, or it shows numbers that are outside the expected range. Replacing the oxygen sensor will probably fix the problem. But if it turns out to be okay, then we should check the wiring.


Some automotive diagnostic tools can also perform a series of other useful functions. One of these features is the ability to clear all the error codes stored in the ECU. This way, if the same code comes back later after the repair, you'll know that the problem was already fixed.


Now that you’ve got the gist of how car diagnostics work, we can finally talk about mileage correction equipment and specifically about mileage correction scanners. At this point, you might have an answer yourself, but let’s reevaluate what we know so far and dive a little deeper.

Automotive scanners are taking information from the car’s ECU in the form of error codes. These codes have their respective interpretation. These tools do not access the vehicle’s odometer.

Thus, how could they know if the mileage was altered or stopped altogether? The best they could do is show an odometer malfunction (but we are yet to encounter such a case). In other words, you cannot detect what has never been recorded.