When a warning light appears on your dashboard, a diagnostic check reads the fault codes stored by the engine management system or other control units. Here is what a diagnostic check actually involves and what it can and cannot tell you.
How diagnostics work
Modern cars have multiple electronic control units (ECUs) that monitor every aspect of the vehicle: engine management, transmission, ABS, airbags, traction control, and many more. When a fault is detected, the relevant ECU stores a fault code and in many cases illuminates a warning light. A diagnostic scanner connects via the car OBD port (a standardised socket usually under the dashboard) and reads these stored codes.
What a diagnostic check can tell you
- Which system has a fault (engine, transmission, ABS, etc.).
- The specific fault code and its description (for example: P0300 random misfire; P0420 catalyst efficiency below threshold).
- Whether a fault is current or intermittent.
- Live sensor data in some cases, showing readings from sensors in real time.
What a diagnostic check cannot tell you
A fault code points to a system or circuit, not always to the specific failed component. P0340 for example means a fault in the camshaft position sensor circuit. That could be the sensor itself, the wiring to it, the connector, or the ECU. Experienced technicians use the code as a starting point and carry out further testing to identify the root cause. A code alone does not mean automatic replacement of the named component.
When should you book a diagnostic
Whenever a warning light appears that you cannot identify, or that stays on after an obvious issue has been resolved. Also before buying a used car, as part of a pre-purchase inspection, to check for stored fault codes the seller may not have disclosed.
Our diagnostic check starts from £45 + VAT. Book online or call 0131 554 3423.