An OBD2 (On-Board Diagnostics II) scanner plugs into a standardised port in every car made after 2001 and reads electronic fault codes stored by the car's ECU (Engine Control Unit). It's a £15–£40 investment that has saved experienced flippers thousands of pounds by revealing hidden faults that no amount of visual inspection or test driving would find. Using one before every viewing should be non-negotiable.
Where Is the OBD Port?
The OBD2 port is standardised at a 16-pin trapezoidal connector, almost always found beneath the driver's side dashboard within arm's reach of the steering column. On most cars it's visible without needing to crouch. On a small number of models it's behind a cover or in the glovebox — a quick search for "[car model] OBD2 port location" will confirm it in seconds.
What the Scanner Actually Reads
- DTCs (Diagnostic Trouble Codes): Stored fault codes from any system — engine, transmission, ABS, airbag, emissions. Even if the dashboard warning light has been turned off, cleared codes leave a trace if they've been reset recently (a "readiness monitor" not complete flag)
- Live sensor data: Real-time engine temperature, fuel trims, O2 sensor readings, MAF values — useful for spotting running issues that don't trigger codes
- Freeze frame data: The conditions (speed, RPM, temperature) at which a fault was triggered — helpful for diagnosing intermittent problems
Red Flag Codes: Walk Away
- P0300–P0308 (Random/Multiple Cylinder Misfires): Active misfires indicate serious issues — worn spark plugs and coils on the cheap end; head gasket failure, injector problems, or timing issues on the expensive end
- P0087 (Fuel Rail Pressure Too Low): High-pressure fuel pump failing on direct injection engines — £400–£1,200 repair
- P0420/P0430 (Catalyst System Efficiency Below Threshold): Catalytic converter failing — £300–£800+
- P0700 (Transmission Control System fault): Any active transmission code on a DSG/CVT/automatic deserves serious investigation before buying
- B codes (airbag/SRS): Active airbag faults mean the car may have been in an accident severe enough to trigger the airbag system sensors — even if the bags didn't deploy
Codes That Are Often Manageable
- P0171/P0174 (System Too Lean): Often a vacuum leak or dirty MAF sensor — £20–£80 to fix in most cases
- P0442 (Small EVAP Leak): Usually a loose or degraded fuel cap — £5–£30 fix
- P0128 (Coolant Temp Below Thermostat): Thermostat stuck open — £80–£150 fitted on most cars
The Cleared Code Tell
When codes are cleared, the car's "readiness monitors" reset. These monitors run self-checks on various systems and take a period of normal driving to complete. A car with multiple incomplete readiness monitors has had its codes recently cleared — the seller is hiding something. A quality scanner (not just a basic code reader) will show you which monitors are complete and which are "not ready." More than two "not ready" monitors on a car should raise immediate suspicion.
Recommended Scanners
For occasional use: the Vgate iCar Pro (Bluetooth, £15–£20) pairs with a free smartphone app (Carista, OBD Fusion, or Torque) and reads codes on virtually all OBD2 vehicles. For more advanced use including manufacturer-specific codes, the Autel MaxiCheck MX808 (~£180) covers all systems including ABS, SRS, and transmission on most car brands. The Bluetooth dongle is sufficient for 90% of pre-purchase checks.