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I'm looking to buy my first car and I went to a second hand car dealership to test drive one of their cars. I've got an OBDII fault code reader but when I wanted to plug it in the man said that in the past other people had plugged those things into cars and it had broken the car, so he refused to let me plug mine in. When asked how he just said that the lights on the dashboard lit up, he wasn't very specific about how it actually damaged it.

I'm wondering if this is a legitimate issue he might have had or if he was just not wanting me to find out about all the faults the car has.

The visual check of the car I did was fine and the tyres including the spare was fine. And when I looked though the service history of the car it all seemed fine to my extremely limited experience and there was no obvious issue with driving the car, this is the only real thing which made me wonder about how good it or the dealership might be.

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    Giant red flag indeed.
    – Jon Custer
    Commented Sep 27 at 17:14
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    Some aftermarket code readers are quite cheap and nasty. For all they know it could be one you hacked together yourself. It might be worth asking if they would allow an independent vehicle assessment, including a diagnostic check. In UK the RAC and AA offer various levels of vehicle check. If they refuse that then I agree with the red flag.
    – Weather Vane
    Commented Sep 27 at 17:26
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    Random "ELM327 clones" bought from your favourite big online marketplace (especially when combined with dubious mobile OBD apps) can indeed mess up a car very easily. It's enough to misdetect the communication protocol to use (there are 10+ different incompatible options) and feed a stream of garbage commands to the ECU to confuse it badly, and you can even accidentally reprogram something important while doing so. Is your OBD reader a certified, professional piece of equipment, or one of the cheap knock-offs?
    – TooTea
    Commented Sep 30 at 12:39
  • Is it not common any longer to let a prospective customer test-drive a car for 15 minutes or so? Hint: Ask a friend to test drive it.
    – Peter - Reinstate Monica
    Commented Sep 30 at 17:22
  • Curious: what is stopping you from using your scanner in the grocery store parking lot down the street during your test drive?
    – spuck
    Commented Sep 30 at 19:26

2 Answers 2

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This is a legitimate cybersecurity issue due to car theft.

Immobilizers and keyless entry systems are programmed through the OBD port. With a scanner, you can simply pair a new key fob to the vehicle, come back and drive away.

Newer cars require registration and an access token from the manufacturer's server to access key and immobilizer functions, but many older cars have no such security.

A no OBD port access policy is present even in quasi-professional settings like auto auctions (see "CAN I REMOVE THE TRACKING DEVICE?").

You will either have to use a mutually trusted mechanic, ask the dealer to do a scan for you with their tool, or merely observe the CEL.

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Absolutely a red flag. If he won't let you use your OBDII scanner, then ask him to bring his out. If he believes the car has no issues, then he shouldn't fear the scanner (yours or his). If he still refuses, walk away. There are better cars and definitely better car dealerships to work with.

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    Still, I wouldn't be surprised to see a car dealer that allows for eg. a third party garage to check the car, with presumably reliable scanners, and still be vary of a chance customer walking in with possibly the cheapest, crappiest AliExpress/Temu scanner doing some damage. If they refuse all solutions, including their own scanner, that's completely different, of course.
    – Gábor
    Commented Sep 28 at 12:58
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    It's the law in the state of California that dealers must allow cars to be inspected by third-party garages - I'm certain that many other jurisdictions have similar laws. @Gábor
    – gparyani
    Commented Sep 30 at 21:26

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