TL;DR: To use an OBD2 scanner, plug it into the car's OBD2 port with the ignition on, let it read the vehicle, then view and note any diagnostic trouble codes before you clear anything. For a fleet, an OBD2 scanner is a first-line triage tool: it tells you whether a fault is something you can handle in-house or one to escalate to a specialist.
Every modern car sold in the UK has an OBD2 (On-Board Diagnostics) port, and learning to read it is the fastest way to cut guesswork when a warning light appears. For a workshop or fleet running mixed vans, teaching the team to use a scanner for first-line checks means a vehicle is not sitting idle waiting for an external diagnosis it may not even need.
Where is the OBD2 port?
On almost all cars and vans it is within reach of the driver's seat — usually under the dashboard near the steering column, sometimes behind a small cover. It is a trapezoidal 16-pin socket. If you cannot find it, the owner's handbook lists the location.
How to use an OBD2 scanner, step by step
- Plug in with the engine off: connect the scanner (or its VCI) to the OBD2 port before touching the ignition.
- Switch the ignition on: turn the key to position II (dash lights on) or, for a running test, start the engine as the tool instructs.
- Let the tool identify the car: a good scanner reads the VIN automatically and loads the right vehicle profile.
- Run a scan: read the stored diagnostic trouble codes (DTCs) and any pending codes.
- Record before you clear: write down every code and its description first — clearing them erases the evidence you need to diagnose the fault.
- Check live data: where available, watch sensor readings (coolant temperature, RPM, fuel trims) to see the fault in context.
- Decide and act: fix the cause, then clear the codes and re-scan to confirm the fault does not return.
Reading trouble codes
DTCs follow a standard pattern: a letter for the system (P for powertrain, B for body, C for chassis, U for network/communication) and four digits. The scanner shows the code and a short description, but the description is a starting point, not a diagnosis — a “lean mixture” code can have several causes, so confirm with live data before replacing parts.
When should you clear codes?
Only after you have recorded them and addressed the likely cause. Clearing codes to make a light go away without fixing the fault simply hides the problem until it returns — often at the worst moment for a vehicle in service. After a genuine repair, clearing the code and re-scanning is the correct final check.
The limits of a basic OBD2 scanner
A basic OBD2 reader is excellent for engine-related codes and simple triage, but it mostly reads rather than commands. It cannot run active tests, code a replacement module, or reach systems locked behind a secure gateway on newer vehicles. When a job needs you to actuate a component or configure a module, that is the point to move up to a professional bidirectional tool — our best car diagnostic tool guide explains the difference, and the bidirectional car diagnostic scanner online guide covers what a bidirectional platform adds.
Staying safe and accurate
- Apply the handbrake and keep clear of moving parts when the engine is running.
- Do not clear codes on a customer or fleet vehicle before recording them.
- Keep the battery healthy — low voltage can cause misleading communication errors.
- For coding or clearing on modern vehicles, make sure the person doing it is authorised and understands the risk.
When to step up: AutomotiveCo Pro
When first-line OBD2 checks show a job needs more than reading codes, the AutomotiveCo Pro Diagnostic Scanner steps up to full-system bidirectional control, active tests, advanced ECU coding, OE topology mapping and secure gateway access for FCA, Renault and VAG (SFD) vehicles, with a J2534 pass-thru VCI over Bluetooth or USB. It is £1645.98 inc. VAT (£1371.65 ex. VAT), rated 4.8 out of 5 from 306 reviews, and includes two years of free updates, a two-year UK warranty, UK-based support and free UK next-day delivery — a practical way to bring more fleet diagnostics in-house.
Move from reading codes to proving faults
Frequently asked questions
How do you use an OBD2 scanner for the first time?
Plug the scanner into the OBD2 port with the engine off, switch the ignition on, let the tool identify the vehicle, then read and record the trouble codes before clearing anything. Use live data to see the fault in context.
Where is the OBD2 port on my car?
On most cars and vans it is under the dashboard within reach of the driver's seat, near the steering column, sometimes behind a small cover. It is a 16-pin trapezoidal socket, and the handbook lists the exact location.
Should I clear fault codes after reading them?
Only after recording them and addressing the cause. Clearing codes to switch off a warning light without fixing the fault just hides the problem until it returns. After a genuine repair, clear and re-scan to confirm.
Can an OBD2 scanner diagnose every problem?
No. A basic OBD2 reader is great for engine codes and triage but cannot run active tests, code modules or reach secure-gateway systems. For those jobs you need a professional bidirectional tool.