Checking a used car before you buy: what a scanner shows

Fault codes get cleared before a viewing. But some traces cannot be cleared — and those are the ones to look at.

A scanner does not replace a lift and will tell you nothing about the body. What it does answer is the one question no visual inspection can: what the control unit knows about this car, and what has been done to it recently.

First: readiness, not fault codes

The most useful screen when buying is not the code list — it is the readiness monitors. The unit runs its own self-tests of the catalyst, the oxygen sensors and the evaporative system. Clearing the codes resets all of them, and they build up again only after tens or even hundreds of kilometres of varied driving.

That gives you a simple reading:

• No codes, monitors complete — the picture is honest.
• No codes, but half the monitors "not ready" — the codes were cleared recently. Shortly before you arrived.

Second: the freeze frame

If a code is present, it almost always carries a freeze frame — the conditions at the moment it was stored: engine speed, temperature, load, vehicle speed. That is the difference between "it came on once in winter during warm-up" and "it repeats on the motorway under load".

Third: live data on a warm engine

Ten minutes with the engine running says more than a list of codes:

1. Coolant temperature. It should reach operating temperature and stay there. Still wandering below it after fifteen minutes means the thermostat — that is code P0128.
2. Fuel trims. A steady positive value at idle points to an air leak.
3. Misfire counters per cylinder. Counters ticking on a warm engine are P0300 and its relatives, even if the lamp is not on yet.
4. System voltage. Usually 13.8–14.6 V with the engine running. Lower means the alternator or the wiring, higher means the regulator.

Fourth: the VIN from the unit

The app reads the VIN exactly as the car reports it. Compare it with the documents and the plate. A mismatch is not always foul play — the unit may have been replaced — but it is precisely the question to ask before the money changes hands, not after.

What a scanner will not see

Honestly about the limits, so there is no disappointment:

• Body, rust, paint thickness. None of that exists in the control unit.
• A wound-back odometer. Mileage lives in the instrument cluster; OBD-II reads it on some cars only, and where it does, it can be edited too.
• The state of an automatic gearbox. Its own fault code is visible, wear is not.
• Airbags. That unit speaks a different protocol, and an ordinary ELM327 usually cannot reach it.

The order to do it in

1. Without starting, switch on the ignition and connect. Look at the codes and the readiness monitors.
2. Start the engine. Let it reach operating temperature.
3. Watch the live data at idle and at 2000–2500 rpm.
4. Save the report. It is useful both for haggling and for the mechanic.

All of the above works with any ELM327 adapter — an expensive one is not needed.

Before the viewing itself, the whole sequence helps: how to check a car before buying — step by step, what to do and in what order.