Check a used car before you buy

The short answer

1. Plug a scanner into the OBD-II port under the dashboard, below the steering wheel.
2. Read the stored and pending codes.
3. Open the readiness monitors. Several monitors "not ready" on a car the seller drives every day means the codes were cleared, or the battery was disconnected, not long ago.
4. Ask the seller why, and scan again after a longer test drive or before you pay.

Why a cleared light still leaves a trace

Your car tests its own emissions systems while you drive: the catalytic converter, the oxygen sensors, the fuel vapor (EVAP) system and others. These self-tests are called readiness monitors (NY DMV). When the codes are cleared with a scan tool, or the battery is disconnected, the car reports the monitors as "incomplete" or "not ready" (Ohio EPA). They go back to "ready" only after the car is driven in the right conditions. Ohio EPA says that takes about two or three days of city and highway driving, and longer on older cars. Georgia's emissions program says one to two weeks, with some highway driving (Georgia Clean Air Force). So a car that shows no codes but several monitors not ready was probably cleared, or had its battery disconnected, in the last few days.

Newer cars also store permanent codes. The OBD rules say no scan tool can erase a permanent code, and neither can disconnecting the battery: the car removes it only after its own self-test for that fault passes (CARB 13 CCR 1968.2, sections (d)(2.5) and (g)(4.4.6)).

What you need

• An OBD-II scanner. Gas cars and light trucks sold in the US from 1996 on have the port.
• An app that shows pending codes and the readiness monitors, not only stored codes. ScanMyCar shows both.
• Ten minutes, and the seller's OK to plug in.

Step by step

1. Find the port. It is a 16-pin socket under the dashboard on the driver's side, usually below the steering wheel, near your knees. On some cars it sits behind a small cover.
2. Watch the light first. Turn the key to on, or press start without your foot on the brake, and look at the dash. The OBD rules make the check engine light come on with the key on and the engine off, to show that the bulb works (CARB 13 CCR 1968.2, section (d)(2.1.2)). If it never lights up, ask why.
3. Plug in with the engine off. Push the scanner into the port until it sits firm.
4. Turn the ignition on, or start the engine. Open ScanMyCar and connect to the scanner.
5. Read the codes. Open Engine Codes and tap Scan codes. ScanMyCar lists stored, pending and permanent codes. Take a screenshot of the list.
6. Open the monitors. Go to More, then Monitors & readiness. The top line says READY FOR INSPECTION, NOT READY YET or CHECK ENGINE LIGHT ON. Under Since codes cleared, each monitor shows READY or NOT READY, and the line under the verdict counts them, for example "3 monitors have not finished since codes were cleared."
7. Take the test drive, then scan again. A fault that was cleared can come back as a pending code during a drive.
8. Unplug with the engine off.

What "not ready" means

"Not ready" is not proof of a trick. New York's DMV lists four causes: codes cleared after a repair, a battery recently disconnected or replaced, a computer that needs a software update, and a problem that is pending but has not yet turned the light on (NY DMV). A recent battery or repair with a receipt is a fair answer.

The car also counts the distance and the warm-ups since the codes were last cleared, and the OBD rules make it report both (CARB 13 CCR 1968.2, section (g)(4.10)). In ScanMyCar Pro, the sensor list shows them as Distance Since Clear and Warm-ups Since Clear. A low distance means the codes were cleared recently.

What to ask the seller

• When were the codes last cleared, and why?
• Was the battery disconnected or replaced recently? Is there a receipt?
• Has the check engine light been on in the last year? What did the shop find?
• Can I scan the car again after you have driven it for a few days?
• Can a mechanic of my choice inspect the car before I pay?

What a scanner does not tell you

A scanner reads what the car's computers report today. It does not show the car's history, title, accident record or past owners, and it does not check the brakes, tires, frame, rust or body. Get a pre-purchase inspection from a mechanic you choose, and get a history report from a separate service.

How ScanMyCar helps

The free app reads the codes, the live sensors and the readiness monitors, and names each code in plain words. Obie, the AI mechanic in ScanMyCar Pro, explains what a code means for this car, how serious it is, and what to ask before you buy.

Questions people ask

Can a seller hide a check engine light?

For a while. Clearing the codes turns the light off until the car finds the fault again. The readiness monitors, pending codes and permanent codes are the trace a scanner shows.

How long after clearing codes do the monitors show ready?

Ohio EPA says about two or three days of normal city and highway driving. Georgia's program says one to two weeks, with some highway driving.

Can a scanner tell that codes were cleared?

Not with a date. It shows monitors that are not ready and, on cars that report it, the distance driven since the codes were last cleared.

Is one monitor not ready a bad sign?

Not on its own. Some monitors need special conditions to run, which is why New York passes 2001 and newer cars with one monitor not ready.

Does this work on every car?

The ScanMyCar scanner works with gas cars and light trucks sold in the US from 1996 on, with a standard OBD-II port. It does not work with electric cars.

Sources

• New York DMV, What do you mean my car's not ready dmv.ny.gov
• Ohio EPA, OBD vehicle readiness fact sheet dam.assets.ohio.gov
• Georgia Clean Air Force, learn about OBD testing cleanairforce.com
• California Air Resources Board, 13 CCR 1968.2 OBD II Final Regulation Order (2015) ww2.arb.ca.gov