Electronic diagnostic: what the onboard computer reads

Electronic diagnostic reads the fault codes stored by the vehicle’s onboard computer (ECU). When the check engine light comes on or a problem appears, the garage plugs in a diagnostic tool or OBD reader to identify the cause. Here is what the computer reads and how to interpret the codes.

Principle

Modern vehicles (from around 2000, OBD2 standard in Europe) have a diagnostic socket (often under the steering column). A compatible tool can read fault codes (DTC), clear codes after repair and show live data (temperature, rpm, airflow, etc.). Each code (e.g. P0420, P0301) corresponds to a family of faults (catalyst, misfire on one cylinder, etc.).

What the garage does

The technician plugs in the tool, reads the stored codes, interprets them with documentation or software, then carries out checks (faulty part, sensor, connection). After repair, they clear the codes and check the light does not come back on. A code alone is not always enough: it points to an area (exhaust, injection, ignition), not always the exact part.

Personal OBD reader

Consumer OBD readers (plug-in or Bluetooth) can read and clear codes at home. Useful for a first idea before going to a garage; avoid clearing codes before the garage has read them, and do not treat a code as certain without further checks.

Order of magnitude

Reading codes at a garage is often included in the diagnostic (€30–80) or free. Repair cost then depends on the faulty part. My Auto Check-up helps you list symptoms and prepare your visit for an initial cost estimate.