What is diagnostics about?
I clearly remember when the only engine diagnostics tool a mechanic carried was a screwdriver, writes John Oxley.
He placed one end on the tappet cover, and the other against his ear (not the pointy end), and in this way could listen to the engine, and if he was very experienced, tell you a variety of things about how it was performing, and if anything was broken.
Quite often the more clued-up mechanics would use a doctor’s stethoscope, and to be quite honest, they got fairly good results.
These days, however, engines are a lot more complex, and it requires an electronic analysis – usually with an instrument called a scan tool – which is plugged into a special port in the car’s electrical system, linked to the car’s electronic control unit computer (ECU).
This is called an OBD port (there are variations called ODB I and OBD II), and the scan tool, which is essentially a computer, performs various tests to learn more about why an engine is not functioning properly.
The engine diagnostic is used to gather data which can be used in the repair of the vehicle, either by directly indicating a fault, or by guiding the skilled technician operating it (they’re no longer called mechanics) to find a solution.
The scan tool does this by interfacing with the ECU, then returning a diagnostic code which can be referenced against a list of codes for vehicles from that manufacturer. If the engine is in good working condition, the device will inform the user. If there is a problem, one or several codes may be displayed.
Although there is a variety of scan tools available, they are not much use without having access to a database of codes. Some scan tools manufacturers maintain their own databases of codes for various vehicles, and the best ones update them continually, often downloading them to the tool by just pushing a button on the scan tool itself.
But it’s a skilled business, and if not done properly, by qualified people, can cause irreparable damage to the ECU and require expensive replacements.