How to check a car before buying, in ten minutes
What the socket shows, what it never shows — and why freshly cleared codes are visible.
Agree on the check beforehand
Plugging into the socket changes nothing in the car, but do not reach into someone's cabin unasked. A refusal to show the socket is an answer in itself.
Look at the readiness monitors
The car tests its own systems over several trips. If almost every monitor is «not ready», the codes were cleared very recently — and not without reason.
Read stored and pending codes
There may be no current codes at all, while pending ones tell what the car has already noticed but not yet put on the dashboard.
Start the engine and watch the live data
The temperature should rise and settle near the working point. Fuel trims stay around zero: a steady plus above ten percent means extra air or not enough fuel.
Check how many units answer
A silent unit is a reason to ask why. Sometimes it is a pulled fuse, sometimes a fault, and sometimes a way to hide a code.
Remember what the socket never shows
Body repairs, the state of the suspension, crash history and the true mileage are invisible here. An OBD-II check is half of an inspection, not all of it.