OBD2 check before buying a used car

Before you go

• Make sure the car has OBD2: in the US, 1996 and newer gasoline cars and 1997 and newer diesels. In the EU, petrol cars registered from 2001 and diesel cars registered from 2004 generally have EOBD.
• Charge your phone, install OBD Master Pro and test your adapter on your own car first.
• Ask the seller for permission to plug in a scanner. It only reads data and takes a few minutes.

Find the port and connect

The OBD2 connector (officially the Data Link Connector, or DLC) is generally under the dashboard on the driver's side, near the steering column. EPA guidance treats the area under the driver's side of the instrument panel as the preferred location and allows a few other locations, so on some models it sits behind a small cover, a panel or in the center console.

Plug in the adapter, switch the ignition on and connect from the app's Connection screen using the generic OBD2 / EOBD profile or the car's brand profile.

Read all trouble codes, not just the light

Read stored, pending and permanent codes. A pending code can be present even when the check engine light is off, like the pending P0420 shown in the video below. Note every code and look it up before you negotiate.

Also check that the check engine light itself works. When you switch on the ignition, the light should come on briefly as a self check and then stay off while the engine runs if no problem is detected. A light that never comes on at all is a reason to ask questions.

Check the readiness monitors

Readiness monitors show whether the car's emission self tests have completed since the memory was last cleared. If many monitors show incomplete, the memory was probably cleared recently, for example after a repair, a code clear, or a dead, disconnected or replaced battery.

That is not proof of a problem, but it means the car has not finished its self checks, so a fault may not show yet. The car generally needs about a week of normal driving to complete them. Ask the seller why, or scan again after the car has been driven.

In California a car with the check engine light on will not pass Smog Check, and too many incomplete readiness monitors also cause a fail. Rules in other states and countries may differ.

What an OBD2 scan can't tell you

• Generic OBD2 covers emission-related systems. ABS, airbag and body modules need brand profiles and a capable adapter, and support varies by car.
• It does not show mechanical wear, rust, accident damage or service history.
• A clean scan is a good sign, not a guarantee. For an expensive car, a pre-purchase inspection by a mechanic is still worth it.

Keep reading

All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides