Using an OBD2 scanner when you buy a used car
A cheap OBD2 scanner or Bluetooth dongle (about NZ$30–$60) reads the fault codes a car's computers have stored. It's the quickest way to find a fault the seller didn't mention.
How to use it
- Do it after the test drive, with the engine off.
- Find the port under the dashboard, usually near the driver's knees.
- Turn the ignition on and read the stored and pending codes.
- Photograph every screen of codes before anything else.
- Check the readiness monitors. Several showing "not ready" or "incomplete" usually means the codes were cleared recently, which can hide a fault.
What the codes mean
Codes starting with P are the engine and gearbox, B the body, C the brakes, steering and suspension, and U the network between the car's computers. Look up any code in fault codes.
Many codes at once
A dozen or more codes across different systems, especially U codes, often point to one cause: a weak 12 V battery, a recent battery disconnect or one failing module. Don't add up a repair for each code. Credov groups codes that share a cause and prices the best case, the most likely case and the worst case for each group.
A code isn't a diagnosis
A code says where the car saw a problem, not which part has failed. A workshop confirms it with a diagnosis before replacing parts.
Questions
Where is the OBD2 port?
Under the dashboard on the driver's side on almost every car made since the early 2000s, often behind a small cover near the driver's knees.
Can a seller hide fault codes?
Codes can be cleared with any scanner, but the readiness monitors then show "not ready" until the car has been driven for a while. Several monitors not ready is a reason to ask questions.