My car has an intermittent fault that cannot be fixed. It has been to the garage so many times now and they cannot find the fault.

I figure there is no point putting more effort into trying to fix the problem, the car works fine - the mechanics have reassured me the car works fine, but occassionally the engine lights come on.

Now after the car goes to the mechanics the lights will stay off for a week or so. I bought a cheap Bluetooth OBD device, but when I use this to reset the codes, the lights will be on within a few minutes.

What could be the differences between the mechanics OBD device and mine? What would I need to look for to find an OBD device that could work as well as the mechanics?

  • Have you tried to reset the OBD2 codes without a scanner/bluetooth device? Here is a quick method. Note that ECU 'tweaks' will also be removed and it may take a few hours of driving before the ECU gets back to running the engine at peak efficiency.
    – user16128
    Dec 17, 2019 at 23:57
  • You might want to post a separate question about your problem and how to fix it @MongusPong, we get a lot of those here and often there's good answers.
    – GdD
    Dec 18, 2019 at 8:31
  • @GdD Good idea. I have posted here. Dec 18, 2019 at 22:26
  • Have you considered just leaving it on instead of continually fighting it? Dec 19, 2019 at 17:04
  • @BrianKnoblauch, leaving the lights on? Yeah they've been on for a few years now, but recent law changes in the uk means it will fail its MOT test with the lights on, so I'm forced to deal with it now. Dec 19, 2019 at 22:34

1 Answer 1


What kind of vehicle is it? Are we talking about the check engine light? Some vehicles have multiple communication channels for diagnosis. On my BMW, you have to solder two pins together to see the body modules, otherwise it's only the engine.

That being said, all scan tools should be able to read and reset the code causing the Check Engine Light - it's actually required by law on modern cars.

Unfortunately OBD dongles aren't always implemented very thoroughly/ consistently. If we are indeed talking about the Check Engine Light, I would go to Walmart and get their basic (~$30) scanner which should do a standard OBDII reset.

If the code resets, and then comes back after a small amount of time, it simply means your dongle is working, but the ECU keeps detecting that fault and it needs to be repaired.

  • Its a Renault Clio 1.2. Dec 17, 2019 at 22:57
  • It picks up the error ok, and does reset it, but just for not as long a time.. Dec 17, 2019 at 22:58
  • 3
    If it picks up the error and resets it, that means it is operating normally. It's coming back because the ECU is still detecting that fault - you need to repair the fault and then reset.
    – Drewster
    Dec 17, 2019 at 22:59
  • 1
    @MongusPong - And if it's coming back on that fast, you should be able to get it to the mechanic, which if experienced, should be able to solve you problem and get the vehicle repaired. Maybe it's not your device which has failed you, but your choice of mechanics. Dec 17, 2019 at 23:02
  • 2
    @Mongus Pong You need to find better mechanics.
    – narkeleptk
    Dec 18, 2019 at 1:00

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