Buying and selling a used car
What should you check before buying a used car with an OBD scanner?
With an OBD scanner you can check whether the car has stored or pending codes, whether the readiness monitors are complete (if not, the codes may have been cleared recently) and how temperature, fuel trims and voltage behave live. It does not replace a mechanical inspection, but in a few minutes it tells you whether to go ahead or ask for more checks.
Reviewed by Daniel Morales, founder of Reprorace SL, on
What do I look at first?
The stored and pending codes of the engine and, if your scanner can reach them, of the other control units (transmission, ABS, airbag). A code does not rule the car out, but you need to know what it means and what the repair costs.
Then the readiness monitors: if several are incomplete, the codes were cleared or the battery was disconnected recently. Ask why.
Which live data helps?
The coolant temperature at a cold start and how long it takes to rise (a stuck-open thermostat shows up), the fuel trims at idle and with a little throttle, and the battery voltage with the engine off and running.
On a diesel, if the scanner shows it, the particulate filter status and the number of regenerations; on a turbo engine, the boost pressure when accelerating on the test drive.
What will it not tell me?
The condition of the brakes, suspension, tires, body, leaks or clutch, or whether the car has been in a crash. Nor the service history or any liens on the vehicle.
The mileage you read proves nothing on its own: compare it with the odometer, the invoices and the inspection records.
How do I do it without upsetting the seller?
Ask before plugging anything in, do not clear codes and do not change settings: read only. Do it with the engine cold if you can, because some faults only show up at start-up.
If the seller is a dealer, ask for a signed diagnostic report; if it is a private seller, suggest doing the scan at a shop.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0420Catalyst system (bank 1): efficiency below threshold
- P2002Particulate filter (bank 1): efficiency below threshold
- P0401EGR A flow: insufficient flow detected
Related symptoms
- Engine warning light on and steady: what it means and what to do
- Blue smoke from the exhaust: the engine is burning oil
- Car using oil: when it is normal and what to do
Learn more
See what your car is saying
If the seller agrees, a shop that works with Diaglana can read the car with the ECO-LITE and issue a signed pre-purchase report, with the mileage stored by each control unit and any recent code clears.
Presale price: €129 · US$129
Frequently asked questions
Is a car with no codes fine?
Not necessarily: check the monitors too. Incomplete monitors can point to a recent clear.
Can I clear a code to see if it comes back?
Not on a car that is not yours. Read only: write down what you see.
Will any scanner do?
For the engine, almost any OBD-II/EOBD scanner. For the transmission, ABS or airbag you need one that talks to those control units on that make.