Buyer’s guide
How to scan a used car before you buy it
Clearing a check engine light takes about ten seconds. Undoing the evidence that it was cleared takes several days of driving — which is what makes a scan worth doing before the money changes hands.
Can you scan a used car before buying it?
Yes. An OBD-II adapter plugs into the port under the dash and reads the car in a few minutes, without needing anything from the seller beyond permission to plug in. Two checks matter most: readiness monitors, which reveal whether the codes were cleared recently, and pending codes, which show faults the car has started seeing but that have not turned the check engine light on yet.
A dark dashboard proves nothing
Any $20 scanner can turn a check engine light off. It does not repair anything — it erases the record and lets the car start its self-checks over. If the fault is still there, the light comes back. The question is whether it comes back before or after you have bought the car.
That is why the useful question is not “is the light on?” but “when was this last reset?” The car keeps an answer to that, and it is not the part a seller thinks to hide.
The two checks that matter most
Readiness monitors — was this cleared recently?
Your car continuously runs self-tests on its emissions systems — the catalytic converter, the oxygen sensors, the evaporative system and others. Each one reports as complete or incomplete. Clearing the codes resets every one of them, and so does disconnecting the battery.
They do not come back quickly. Finishing the full set takes a drive cycle that typically means days of ordinary driving. So a car offered for sale with most of its monitors still incomplete was reset recently. That is not proof of a hidden problem — a new battery does the same thing — but it is a fair question to ask, and the answer tells you something about the seller.
Pending codes — what has the car started to notice?
Most faults have to happen twice before the car turns on the check engine light. The first time, the fault is recorded as pending. If it happens again on the next drive cycle, it becomes a stored code and the light comes on.
A pending code is therefore a problem in progress that has not surfaced on the dashboard yet. It is the difference between finding out now and finding out three weeks after you own it.
The scan, step by step
Find the OBD-II port
Every car sold in the US since 1996 has one. It is a trapezoidal 16-pin socket, almost always within arm’s reach of the steering wheel — under the dash on the driver’s side, sometimes behind a small cover or above the pedals.
Plug in the adapter and turn the key to accessory
The engine does not need to be running to pull codes, but it does need to be running to read live sensor data. Turn the key so the dash lights come on, then let the adapter pair with your phone.
Check the readiness monitors first
This is the check most buyers skip and the one hardest for a seller to defeat. Monitors are self-tests the car runs as it drives. Clearing the codes — or disconnecting the battery — resets them all to incomplete, and they only finish again after a full drive cycle, which usually takes days and dozens of miles of mixed driving.
Read stored codes and pending codes
Stored codes are confirmed faults, and they are what turns on the check engine light. Pending codes are faults the car has seen once but not yet confirmed, so the light has not come on. A pending code is an early warning, not a false alarm.
Take it for a drive with the adapter still plugged in
A short drive lets you watch live data under load and gives any intermittent fault a chance to reappear. If the seller cleared the codes before you arrived, a fault that is still present will often set again — sometimes on that first drive.
Treat what you find as a question for a mechanic
A scan tells you which system is unhappy. It does not tell you which part failed or what the repair will cost on this specific car. Use it to decide what to ask about, what to negotiate on, and whether to pay for a pre-purchase inspection.
You need an OBD-II adapter to do any of this — the phone talks to the car through it. Which OBD-II adapter to buy.
What a scan will not tell you
An OBD-II scan reads what the car’s computer has noticed, and the computer is mostly watching emissions-related systems because that is what the standard requires. It knows nothing about brake wear, suspension, tires, rust, frame damage, accident history, or how the transmission actually feels on a hill.
A clean scan is a good sign, not a clean bill of health. Treat it as the cheap check that tells you whether the expensive check is worth paying for.
This page is general guidance — it describes what these checks mean on any car. What a specific code means on a specific engine, with that car’s mileage and symptoms, is a different question, and it is the one worth getting right before you negotiate. Look up a specific trouble code.
Run the scan on your phone
Reading stored codes, pending codes and readiness monitors in Kaiju Mechanic is free — no subscription, no account tier. You just need an adapter. When you find something and want to know what it means for that particular car, the AI diagnosis is there.
Ready?
Your car deserves better
than guessing.
Download Kaiju Mechanic free and start your first AI conversation in under a minute. No appointment. AI chat works without an adapter — live sensor reads use any standard ELM327 you bring. Just answers.