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Lately I've been getting interested in connecting to my car's computer and seeing some diagnostics information. So I need to get an OBD2 adapter (Bluetooth probably, to pair with my phone). But I've heard a rumour that some OBD2 adapters are able to connect to more cars (and/or get more info out of them) than others, and, of course, the difference reflects in price, with the better ones easily costing 10 times as much as the cheap ones.

Now, I'm a bit surprised by this. OBD2 is a digital connection and fairly well standardized, so the adapter itself should be quite simple. All the magic should happen on the software side where, yes, different vehicles might have different vendor-specific extensions to the protocol which the software would need to know. But the adapter itself should be the same for all vehicles, no?

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  • I researched and bought a soecific unit to match my car - one who others recommended independantly of the company advertising. And their adverts proved true.
    – Solar Mike
    Oct 18, 2021 at 9:31

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The "adapters" aren't just dumb transmitters, they're actually a microcontroller running it's own software that implements the OBD2 protocols and then communicates with another device over Bluetooth, WiFi, etc.

OBD2 is a standard though and every device which correctly implements the protocols and commands for it should work the same and on every car made since OBD2 became a requirement. However as anyone who's purchased/used a variety of the cheap "ELM327" dongles that are ubiquitous on eBay, Amazon etc for about a tenner will tell you - the practice doesn't match the theory. So why not?

Well the problem mainly comes from the fact that market is absolutely flooded with cheaply made copies of the real ELM327 device, it used off-the-shelf hardware and proprietary code. Cloners either pirated the original binary from ELM, reverse engineered and bastardized it or created their own shoddy implementation and then gleefully engaged in a headlong race to the bottom by skimping on everything and cutting every corner. Cheap components, shoddy assembly and zero QA.

I've bought a few of these over the years (usually because I keep losing them) and even though I've done my best to avoid the ones that wave the biggest warning signs probably only 60-70% have worked at all, and the rest have all been a bit picky about what cars they would actually connect with. At one point I had two ostensibly identical adapters, bought from the same seller around the same time - one would work fine in one car but not the other and vise versa.

I've heard a rumour that some OBD2 adapters are able to connect to more cars (and/or get more info out of them) than others, and, of course, the difference reflects in price, with the better ones easily costing 10 times as much as the cheap ones.

This is also true.. in addition to the standard OBD2 functionality that every manufacturer implements there's often manufacturer-specific functionality that is available via the OBD port and this is what can provide access to proprietary systems and data in the car. Assuming the physical adapter correctly supports all the OBD2 protocols then most of the "magic" in terms of the extra functionality comes from software on the connected device (PC, phone etc), of course many of the providers of this software also provide adapters and generally have the software written so as to work with their adapter and not generics.

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OBD2 defines both a physical interface (i.e. the part that you plug in to) and an electrical interface.

All cars with OBD2 implement these features. There are, however, manufacturer-specific extensions possible and there is where the cheaper units differ from the professional units used by service centers.

The cheaper units that you buy at auto parts stores for US$100 or so implement the base OBD2 features and may implement some manufacturer-specific features for some popular makes.

The professional-level models almost always have a subscription service that provides them with updates to handle newer models and less common makes so that they can perform more services. These units are quite a bit more expensive and the subscription for updates is also more expensive.

Then there are models specifically commissioned by a manufacturer for their servicing dealers. These are rarely sold to anyone but their own dealers and these have additional features that are not available to unauthorized services centers. That may include features such as firmware updates and diagnostics beyond what even the "pro" level units can do.

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  • OK, but if I buy an OBD2<->Bluetooth adapter, then the adapter itself can be as cheap as it likes, as long as it passes the signal through properly, and the rest is handled in the PC/phone software, correct?
    – Vilx-
    Oct 18, 2021 at 10:27
  • Well I suppose it depends on what you mean by "as cheap as it likes". Some cheap hardware is just junk.
    – jwh20
    Oct 18, 2021 at 10:30
  • Of course. I don't mean such crap that doesn't even perform what it is supposed to do. I mean, an adapter that just forwards the OBD2 protocol via Bluetooth and back and nothing more. That would not need to cost 100$, no? After a bit more research I found that a popular interface for this is "ELM327" and adapters like that retail for less than 20$. And most Android apps seem to support it. There is a risk that they could be knockoffs, but as long as it behaves identically to the original, I think I should be able to see everything there is to see, no?
    – Vilx-
    Oct 18, 2021 at 10:38
  • I've found that "ELM327" adapters are rarely what they are advertised to be. If you're shopping in the US$20 range you are looking at "cheap junk".
    – jwh20
    Oct 18, 2021 at 10:41
  • So... I take it then that they fail even at the basic task of forwarding data between the two interfaces?
    – Vilx-
    Oct 18, 2021 at 10:43

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