How to check a used car's fault codes before you buy
Ten minutes with a £20 adapter tells you what the dashboard won't — including whether the codes were wiped before you arrived.
A warning light that's off doesn't mean a car has no faults. Lights only cover some faults, pending faults never light one, and anyone can clear stored codes with a cheap reader the morning of a viewing. The car's own computers remember more than that, and a phone can read it.
What you need
- A Bluetooth Low Energy OBD-II adapter — see which adapters work with iPhone.
- An app that reads more than stored codes: pending and permanent codes, readiness monitors, and the "since cleared" counters.
- Permission from the seller. Plugging in is harmless and reads only; say what you're doing.
Step by step
- Find the port. Every petrol car sold in the EU since 2001, every diesel since 2004 and every US car since 1996 has one, usually under the dashboard on the driver's side, sometimes behind a small cover.
- Ignition on, engine off to start with. Check every warning light on the dash comes on and then goes out — a light that never comes on may have had its bulb removed.
- Plug in and read. Note any stored codes (confirmed faults), pending codes (seen once, not yet confirmed — often the earliest warning) and permanent codes (which clearing can't remove until the car has proved the repair itself).
- Look at the readiness monitors. The car runs self-tests on its catalyst, oxygen sensors, evaporative system and more as it's driven. After codes are cleared they all reset to "not complete" and take days of normal driving to finish.
- Read the "since cleared" counters. Distance and warm-ups since codes were last cleared start again from zero. A car with 80,000 miles on it that has done 12 miles and 2 warm-ups since its codes were cleared has been cleared very recently.
- Check the VIN. The engine computer reports the car's VIN; compare it with the logbook and the plate in the windscreen. Other modules report theirs too, and one that disagrees has come from another car.
- Then test drive, and read again afterwards. A fault that was cleared often comes straight back as a pending code within a drive.
What the results mean
| You see | It suggests |
|---|---|
| No codes, monitors complete, hundreds of miles since cleared | Nothing hidden. A good sign. |
| No codes, but monitors incomplete and few miles since cleared | Codes were cleared recently. Could be a new battery or an honest repair — ask why, and ask for the invoice. |
| Permanent codes with no stored ones | A fault was stored and has since been cleared, and the car hasn't yet confirmed it's fixed. |
| Pending codes | Something the car has noticed but not confirmed. Look each one up — fault codes explained. |
| Distance driven with the check engine light on | The light has been on for that far since codes were last cleared. |
Recently cleared isn't proof of anything. A battery change, a garage visit or a disconnected battery all leave the same marks. It's a reason to ask questions, not an accusation.
Free to connect and see your car live; recording, fault codes and the used-car check are part of an OBDTrip subscription.
Codes worth walking away over
Misfires (P0300–P0306), timing correlation (P0016), overheating (P0217, P0118) and gearbox faults (P0700, P0740) can mean expensive work. A catalyst code (P0420) is worth pricing into an offer. Most emissions and sensor codes are cheap.
OBDTrip's used-car check does all of this in one go
Plug in for a few minutes and OBDTrip reads every code, the self-tests and the since-cleared counters, says plainly whether the codes appear to have been cleared recently, checks every module's VIN against the others, and saves a report you can look back at after the test drive. The car isn't added to your vehicles, and nothing is recorded, synced or sent.
