Checking a used car over OBD before buying
In a few minutes, an OBD2 adapter shows whether the engine computer has stored faults, and whether someone cleared them recently. It doesn’t replace an inspection by a mechanic, but it tells you what to ask.
Updated
Before you start
Ask the seller first. You need the ignition on; the engine can stay off for most of the checks. Plug the adapter into the socket under the dashboard and connect Read My Car.
Check that the warning light works
With the ignition on and the engine off, on most cars the check engine light comes on as a bulb check, and goes out once the engine runs, unless there’s a fault. If it never comes on at all, it may have been disconnected: a reason to ask why.
Read all the fault codes
- Stored
- Confirmed faults. They turn the check engine light on.
- Pending
- Faults the car has seen but not confirmed yet. Worth asking about.
- Permanent
- On cars that have them: faults the car hasn’t yet confirmed as repaired. They can’t be cleared with a scan tool.
Read My Car explains each code and how urgent it is. Look the codes up and ask the seller about each one.
Were the codes cleared recently?
Clearing codes hides them until the car finds the fault again. Under Vehicle, “Since faults were last cleared”, Read My Car shows the distance driven, the engine running time and the number of warm-ups since then, where the car reports them.
A very low distance or only a few warm-ups means the codes were cleared recently, or the battery was disconnected, which resets them on many cars. That isn’t proof of a hidden fault, but it is a reason to ask why, and to check again after the car has been driven more.
Emissions readiness points the same way: several self-tests not complete usually means the codes were cleared not long ago, or the battery was disconnected; on cars used mostly for short trips one or two can stay not complete.
Distance with the light on
Many cars record how far they’ve been driven with the check engine light on. Read My Car shows it under Vehicle when the car reports a distance. A large number means the car was driven for a long time with a fault. Clearing the codes resets it to zero as well, so a zero on its own proves little.
Compare the VIN
Read My Car shows the vehicle identification number (VIN) the engine computer reports, where the car reports it; many older cars don’t. Compare it with the VIN on the car, usually visible through the windscreen or on a door pillar. The VIN on the car must match the registration documents: if it doesn’t, don’t buy until it’s explained. If only the engine computer’s VIN differs, the computer may have been replaced: ask why.
What OBD doesn’t show
- Generic OBD2 reaches the engine and often the gearbox. ABS, airbag, body and comfort faults need a tool for the make.
- Mechanical wear, rust, accident damage and the service history aren’t in the engine computer.
- It can’t confirm the odometer reading. The distances Read My Car shows count from the last clear, not from new. In the Netherlands, the NAP mileage record is the place to check.
- A clean scan doesn’t mean the car is fine. For a car you’re serious about, have it inspected by an independent mechanic.
Questions
Can a seller hide fault codes?
They can clear them, but clearing resets the counters since the last clear and the readiness self-tests, which Read My Car shows. Permanent codes, on cars that have them, can’t be cleared with a scan tool.
The distance since the codes were cleared is very low. Should I walk away?
Not necessarily: a battery change can reset it too. Ask why, and if you’re still interested, read the codes again after the car has been driven a few days, or have it inspected.
Does a clean scan mean the car is fine?
No. OBD only shows what the engine computer, and often the gearbox, have recorded. Have the car inspected by a mechanic as well.