Don't Miss

Car immobilizer system checking with an oscilloscope

By on December 10, 2018
Car immobilizer system checking with an oscilloscope











In this video tutorial I will show a really basic setup and “how-to” steps on a car immobilizer system. The car is an old car from the end of ’90. but I found it really good for and experimental purpose, to show  some practical steps  how this system is working and how to use an oscilloscope to check what is wrong.

On this car, the immo light illuminate on the dash and the car cannot start. Some technicians tried to figure out what is the real problem but without success. The immobilizer system is a really simple one in this car because this type of immo stuff is a one wire communication system. That means, through one wire is the communication build up between the immo box and the ecu of the car.

Nowadays modern cars has a much more complicated system, where the communication goes through a so called CAN bus communication protocol and that is much harder to trace but also possible.

Our goal in this article is to realize how to check does any communication is relaying between the car key and the ecu. Many times technicians can’t really decide what is wrong, the transponder in the key, transponder amplifier, immobilizer box, ecu or wiring. Here is picture about the scoping, in this case we deal with a broken ecu *dump because the immo box sends non-stop an authorization request to the ecu but no response from the ecu.

car immobilizer repair

* “dump”  is the name of the content of some sort of memory device like a firmware or eeprom content.

Let’s watch the video and I hope you will enjoy it.

I hope you enjoy this tutorial and will save lot of repair time.

This article was prepared for you by Christian Robert Adzic from Novi Knezevac-Serbia.

Please give a support by clicking  on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.

P.S-  If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!

Note: You can check his previous post in the below link:




  1. Parasuraman S

    December 10, 2018 at 10:33 am

    Vow! Great! That's a lot of knowledge we got without craning our neck! Many thanks!

  2. Albert van Bemmelen

    December 10, 2018 at 3:36 pm

    After reading your very short article I wonder if you managed to fix the car's ECU communication problem? And if you did... how that scope diagram would look like to confirm this conclusion? And where exactly you sampled the immo signal from? Interesting to see you used a modern Siglent logic analyzer.

    • Christian Adzic

      December 11, 2018 at 3:28 am

      Thank's for supporting my article.
      I probably won't fix this type of problem, because the car is too old,
      but that would depend on the customers wish.
      The main interest to make this article for me was to show how to use
      an oscilloscope to check the communication between the ecu and the immo system.
      I decide to do this on this type of car because it is an old car where are not so much thinks in the system and I assume it would help people to more understand some basics than to repair it.
      To repair let's say this car, I probably deal with a corrupted eeprom/flash content and I have to reflash that memory device with a virgin content ( virgin content are content where no keys were added before and/or no setups etc. ) and add (program) new key to the car and probably make some software tweaking but not every car need that...
      Another way would be to make immo off. That means, to hack the eeprom/flash content of the ecu in a way so the ecu newer ask again for a valid key from the immo box and/or the ecu is in an "unlocked" mode.
      Another way for repairing this kind of problem is to buy an ecu, immo box and key with a key barrel from a donor car.
      But take care with this:
      1. People who sell this kind of stuff may have a copy of a key programmed inside of the ecu and immo box, so they could probably get access to your car.
      2. The above listed parts MUST match each other, that means, all that parts MUST be from the same donor car.

      Logic analyzer:
      Not a bad tool to use it in such a situation, but after several repairing, you do not need really accurate data snaping from the system, you will recognize the patterns on the scope.
      A much better tool what I would use is a CAN-Bus sniffer with a good software to interpret the communication traveling through the CAN-bus network.
      Modern cars from maybe 2005 and up are equipped with CAN-bus networks and they use this network to communicate between computers in the car.
      Of course, they are other protocols also in the same car too like ISO9141 or LIN bus.
      Just to stay simple, the main principle is the same.
      I hope my video will help somebody to understand some basics in this field.
      My best regards.

      • Albert van Bemmelen

        December 11, 2018 at 4:09 pm

        Thanks Chris. Understood. Using a Logic Analyzer also would present the communication data in real-time while investigating the various bus signals. Which is a great improvement over using a standard oscilloscope which needs to be interpreted by the user correctly first. Even a modern multimeter like the Mooshimeter that is equipped with Bluetooth and Android plus iOS support can improve investigating those signals greatly! And the Mooshimeter can also record and measure current and voltage at the same time too.

  3. Robert Calk Jr.

    December 11, 2018 at 2:26 am

    Thanks Chris. I have never worked on stuff like that yet.

  4. moshe

    December 13, 2018 at 5:19 am

    Thanks Chris...thoroughly enjoyed it (& your other articles)...a great 'tutorial'!!!

  5. Peter Paul Sultana

    December 13, 2018 at 9:35 pm

    Very good tutorial.
    I have a VW Polo model 6N1 (1998)with a faulty fuel guage and speedometer but the odometer (mileage) is working fine. I have to replace the instrument cluster.I found a second hand one but obviously the odometer reading is different from mine. Is there a way that I can use to change the odometer reading? Is the eeprom that saves the reading in the instrument cluster or in the ECU. If this is the case I can replace the eeprom. Thanks for your reply.

    • Christian Adzic

      December 16, 2018 at 1:57 am

      You say Polo 6N1 hmmm..
      I can't remember now how that is on that type of instrument cluster, but if I remember me correct you should have an eeprom something around 93xx or so.
      You have to transfer the data from the old instrument cluster to the "new-used" one. That should be enough to make working your car and dash.
      You could also physically change the eeproms too.
      Take care to use the same instrument cluster from the same maker.
      I assume your cluster could be a TRW cluster if so, use a TRW cluster but it could also be a VDO too. Don't mix them up.

      My best regards.

    • Christian Adzic

      December 16, 2018 at 5:53 pm

      I got a new idea, why you do not check the not working gauges driver ic or that little motors one of them is not working. Most time the gauge motor is the problem. I repaired it a lot of time in cars. Just changed them.
      I will these days write an article about how to repair a dash, I have an old dash from a Golf3 or so somewhere, it will serve nicely for this kind of article.
      Maybe you can get some idea how to solve your problem...
      Stay tuned on this blog site pls...

      Mu best regards.

  6. Justice

    December 14, 2018 at 4:49 pm

    Mmm nice work Christian keep up posting such interesting articles.


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.