How does a car diagnostic tool work?


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With a diagnostic device, you can really get a feel for your car. You can answer questions such as — which faults and problems have occurred in my car? How much fuel does my car really use? What is the exact performance data of the car? A diagnostic device can read out hundreds of such readings and analyze them in more detail. There are basically two types of diagnostic tools. One is an external diagnostic device such as those used in car workshops, the second option is a mobile phone app in conjunction with an OBD adapter. We discuss both options below. 

So how does such a diagnostic device actually work? In our detailed report, we show you how well the devices work, what you have to pay attention to when buying them, what OBD2 means, and which devices and apps are recommended. Also for the technically inclined among you, we’ll delve into some interesting details. 


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What types of diagnostic devices are available?

Basically, there are three different ways to get the diagnostic values from your car. In all three cases, a connection to the vehicle is established via the OBD2 interface … but more about this later. Here are three options:

  1. Fully-fledged diagnostic devices with their own monitor
  2. Diagnostics via software on a laptop/PC
  3. Mobile apps


Fully-fledged diagnostic devices with their own monitor

Just like the professionals in a car repair shop, you can also work at home with a full-fledged diagnostic device. By full-fledged we mean that it is a kind of computer with its own monitor and software. Most are small, handy devices about the size of a smartphone. Others can be the size of a navigation system or tablet. 

Since these devices are built for one purpose only, they often have the most functionality (compared to an app, for example). But they are also the most expensive of the three — after all, you have to buy the integrated computer with it. They can cost up to €2,000, though very inexpensive devices (which are not recommended) can be bought from around 30 Euros. In the middle there are those around 400 Euros, which usually do a solid job.

From our point of view, the devices costing €400+ only make sense for those of you who are in the used car business or something similar. For those who only want to diagnose their own vehicle, the purchase may be a little too pricey— even if you can certainly get a lot of information from your car.

Diagnostics via software on laptop/PC

As the term already suggests, it is not a separate device for diagnostics, but rather the necessary software for it. In order to be able to perform the diagnosis, you must have a laptop nearby on which you have installed the software. The PC can then connect to the vehicle via Bluetooth or WLAN using an OBD2 adapter. A connection via USB cable is usually also possible.

We see two problems with this option. First of all, you always have to have a laptop in your car if you want to read and/or analyze data. This makes the recording of real time data a bit annoying. On the other hand a regular update of the software is not guaranteed and so you might miss important information, or in the case of new vehicle models you might not be able to recognize them. 

Mobile Phone Apps

Our favourite option is using an app. You will find a variety of apps for iOS and Android to diagnose your car. We see three big advantages with this choice. First, the apps are usually much cheaper than the other options. Second, you are always up to date, because they are continuously updated in the background and third, they’re easy to use.

You already know how your smartphone works. Again, the connection to the car is established via Bluetooth or WLAN and the OBD2 adapter. So you can easily call up real-time data while driving and save it for later analysis. 

Why do you need an OBD2 adapter?

OBD means on-board diagnosis and has been a worldwide standard since 2001 (in the USA since 1996) for the connection to the control units of the vehicle. The functions of OBD are the following:

  • Continuous monitoring of all exhaust relevant values.
  • Recording and reporting of significant increases in emissions.
  • Saving data in case of errors (error codes).
  • It provides an interface for reading the stored data and coding new settings.


With the adapter you get access to this data. It looks like a thick 16-pin connector. Of these, 7 pins are reserved for manufacturer-specific purposes and are not relevant for OBD. This adapter also contains the necessary Bluetooth or WLAN connection to communicate with the mobile phone or the diagnostic device (if this does not work via a cable).

Car coding 

With OBD devices, data can flow in both directions. Data can therefore be retrieved and/or sent back to the control unit for coding or deleting error codes. In principle, the following basic functions are predominant in communication: selection of the diagnostic operating mode, authentication mechanisms, reading and writing of memory areas, control of ECU actuators, access to internal error memories, freely definable diagnostic routines, data transfer for reprogramming, control of communication behavior, and ECU reset. 

When choosing an adapter you should make sure that an original ELM 327 chip is used. This chip is farther along in development than older chips and is able to support additional CAN protocols and to work with 500 kbps. Cheap adapters often contain replicas which can lead to errors and do not work at all. A proper OBD2 adapter can essentially not be found for under €30.

What is hidden behind the error codes?

There are thousands of error codes. Most of them are standardized and apply to all manufacturers. But others are manufacturer-specific. The software of the diagnostic devices or apps knows many of the codes and provides them to you with clear explanations. 

But it can’t hurt to know how the codes are structured and to dive a little deeper into the matter. Each error code consists of 5 digits — one letter at the beginning and four numbers. The structure is as follows:

What exactly does a diagnostic device do?

No matter whether it is a full-fledged diagnostic device, software on your PC, or an app on your mobile phone — basically they all work the same way. First of all, the OBD2 interface is used to read the data from the vehicle’s control unit. Then what happens next depends on your device. In some cases only simple error codes are displayed in plain text, in other cases you get a graphically presented analysis of all data. 

The better devices and apps also allow you to retrieve data in real time and follow all values live. Sometimes the data can even be projected onto the windshield via the Head Up Display (if your car is equipped for it).

Since there are sometimes a lot of manufacturer-specific codes and values, some diagnostic devices cannot cope with them properly. Only very few really have the huge database necessary to actually understand all (or at least most) codes. But there are also app developers, like Carly for example, who have an app compatible with several manufacturers, so that they can adapt very precisely to the specifications of these individual models. 

What does ISO 15031-5 or SAE J1979 say?

This international DIN standard regulates the communication between a vehicle and an external exhaust emission-relevant diagnostic device. In plain English: DIN has defined the language between the diagnostic device and the vehicle. The following diagnoses are theoretically possible if the app, the diagnostic device, or the software offers it. You can see this list as a catalogue of possibilities and choose the app or device that actually enables most of them:

  • Query current diagnostic data – over 135 parameter IDs can be queried and displayed in real time, plus engine indicator light status, readiness codes, number of error codes stored in the system, type of OBD system.
  • Query Freeze Frame Data – emission relevant data .
  • Receive and delete error codes.
  • Querying values of the lambda/oxygen sensor.
  • Query on-board monitoring test results for systems that are not continuously monitored, such as the catalyst and degassing system.
  • Query of vehicle information such as VIN and calibration identifications.
  • Query of the permanent diagnosis error codes – long-term errors that are checked and monitored by the system.

What is a CAN bus?

Until 1996, the length of wiring harnesses in a vehicle could easily be up to 2 km. To reduce this, a bus system was introduced — the data highway in your vehicle, so to speak. 

A bus is a system for data transmission between several participants, which is carried out via a common transmission path. If at a certain point in time data is transferred between two areas, other areas can announce that they need the bus, but they have to wait for it. 

The CAN bus (Controller Area Network) is a special bus developed and standardized for vehicles, which has been able to massively reduce the length of wiring harnesses. The data requests are processed by the bus bit by bit. It works so fast that there are very short waiting times. Such a bus can work either on a copper line or via fiber optics. 

In this way, the bus ultimately ensures that the data is made available at the control unit and, in the event of reprogramming (coding), that the data is transferred to the appropriate locations. 

How exactly does communication between the diagnostic device and the vehicle work?

This is a section for the car freaks among you 🙂 You have to imagine the communication between the ECU and the diagnostic device or the app or software as a question and answer game. Standardized self-contained data units (data packets) are sent. Most data packets consist of the information to be sent (data bytes), important address and management information (header bytes) and a final checksum byte. 

Diagnostic device Auto how does the

Header bytes could, for example, have information about the address (where the data comes from and where it should go), the length of the following data bytes or the priority. During programming the ELM chip (see above) already takes over the task to generate the header and the checksum byte. This shows how important it is to work only with an original ELM and not with a cheap counterfeit product. 

OBD2 is able to operate with purely functional addresses. So you don’t need to know the exact address of an ECU (like engine control unit, transmission control unit, or brake control unit). In this way, most diagnostic devices are compatible for many (or all) vehicle models. However, if the software/device/app is specially developed for a manufacturer, absolute addresses are possible and thus a much more accurate and faster analysis.

What is OBD3?

OBD3 is still a dream of the future — but it could arrive faster than we think. Imagine the following situation: You get a warning signal that your vehicle’s emissions are too high. You only have six months left until the next exhaust emission test and the general inspection. So why go to the garage now to have it checked? The six months do not matter now. 

If OBD3 was already standard, it would work completely differently. The car would then automatically send a message to the relevant authority — via SIM card or satellite. If you want to avoid the fine, you have to have the fault rectified within the period of time allowed. OBD3 is already being discussed in the USA. Whether or when it will be introduced in Europe is still unclear. 

Our Recommendations

Finally, we would like to recommend some devices (full-fledged diagnostic devices and apps) that we have examined in an earlier test. 

Diagnostic Devices

Autel MaxiCheck MX808 automotive diagnostic device

This really great device from Autel is in the semi-professional range and costs around €500. It is a handheld device with a color display and a huge range of functions. All common car brands and models are supported and automatically recognized by the device. 

Here is an outline of the possible functions: Reading and deleting error codes in all available systems of the vehicle, display of live data as text, graphics or analogue for simple data checking, coding of injectors after replacement, teach-in of steering angle sensor (SAS), resetting oil and service intervals, and much more.

LAUNCH X431 V+ (Pro3)

Another device in the semi-pro to professional league. At just under €1,000  you get quite a lot for your money. Apart from the very robust design, it offers a very long battery life, wireless Bluetooth diagnosis, easy operation via touch screen, and a huge scope of delivery with all imaginable cables and connections for practically every (even older) vehicle — making it without a doubt a worthy investment. 

ICarsoft CR Pro

This diagnostic device is available for around €250. It supports more than 40 vehicle makes and models and processes data such as: engine and transmission control unit, ABS, airbag, electronic parking brake, and central locking. The large 4” screen makes for a very pleasant working environment. The updates are free of charge.


Carly App

Over 1 million installations so far speaks for itself. Carly impresses with its very clear menu navigation, its large range of functions, and the fact that there are special apps tailored to various manufacturers.. A nice feature is the “Health Check”. With this value you can immediately see how your car is doing. It is also possible to read out real-time data. The coding of the on-board computer, the navigation, the lights, the brakes, the window setting, and more are very easy. 

Carista OBD 2

The free basic version can already be used for resetting error codes. In the full version you can fully customize the menu and perform extensive diagnostics. The app is constantly updated and has already been installed more than 500,000 times. The readout of real-time data is unfortunately not possible. 

Torque Pro OBD 2 Car

This app impresses with its enormous range of functions, which also includes real-time data analysis, head-up display mode, dynamometer, and GPS/OBD speedometer including tracking. The very special highlight, however, is the possibility to upload your data to the developer’s server, where you can perform even more detailed analyses. The data can also be sent by e-mail or saved as a CSV file.

Keep in mind, that every car has different software and modules built-in. Therefore specific features will vary with every model.

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