Buying a used car: scan it with OBD2 before you sign

Why scan before buying

The engine warning light may be off because everything is fine… or because it was cleared an hour before you arrived. A scan reveals what the eye cannot see: pending codes, a DPF or catalytic converter code, intermittent misfires, an abnormal charging voltage. Ten minutes to avoid months of regret.

The 6-point checklist

DiagAuto AI offers a “Buying a used car” path: a checklist to follow during the viewing, then a check-up that reads the codes, the emissions checks, the distance and time since codes were cleared, and the recorded mileage when the car provides it. The points below cover the essentials.

1. Warning lights with the ignition on
All the warning lights should come on with the ignition, then go out once the engine is running. An engine or airbag light that never comes on with the ignition = bulb removed.

2. Read all the codes
With DiagAuto AI: confirmed, pending and permanent. A permanent code does not disappear with a simple clear command: it stays until the control unit has verified the repair itself (on cars that support these codes). Look them up in the fault code dictionary and ask the AI to explain them on the spot.

3. Emissions checks and distance since codes cleared
If the car is driven every day, the emissions checks are “ready”. If almost all of them are “not ready”, or if the distance driven since the codes were cleared is very short, the codes were cleared recently, or the battery was disconnected. Ask why.

4. Sensors with the engine warm, at idle

• Temperature: should reach 85 – 100 °C.
• Fuel trims between −8 % and +8 % (otherwise).
• O2 sensors: upstream oscillating, downstream steady (otherwise, catalytic converter).
• Voltage: 13.8 – 14.6 V.
• On a diesel, an intake pressure close to atmospheric pressure (around 95 – 105 kPa) and a higher air flow than a petrol engine at idle are normal.

Normal values are detailed in the 10 sensors to watch.

5. Diesel-specific checks
The DPF differential pressure and soot load are not part of the standard OBD2 data the app reads: you need the manufacturer's tool (garage, professional inspection). With the app: DPF codes (guide), fuel rail pressure (actual value, when the car provides it), and a hill test to check there is no limp mode. A flashing glow plug light = walk away or negotiate hard.

6. Test drive with the sensors on screen
Firm acceleration, with a passenger watching the screen: on a turbo, intake pressure rising clearly, no misfire, no smoke (colours). After the drive, read the codes again: a code that appeared during the drive is an active fault.

Warning signs

• A seller in a hurry who refuses the scan.
• A car that is already warm when you arrive (hides a hard start or smoke when cold).
• Emissions checks not ready + “I changed the battery last week”.
• Codes P2002/P2463, P0420, P0087, P0299 present: potentially expensive repairs, to be priced before you negotiate.

And after the purchase

Do a full scan in the first week and save it in the DiagAuto AI history (export as a PDF report with Premium, or take screenshots): it is your baseline record for the future.