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The ODB diagnostic port is used by garages and dealerships to connect your car to their computer systems, checking for any reported errors as well as check various engine values.
Just to check and clear an error message can cost £30+, yet ODB readers can be bought for not much more than £10 and phone apps can then be found for free to then display the output.
Some car manufacturers make the port difficult to get to, often they’re found under the bonnet. But on the 2017 model Mazda 3 it’s ridiculously easy to access.
Yep, that white socket is it. Plug your ODB reader into that, pair up your phone via app and you can review car data until you’re delirious with joy.