The words you meet in fault code descriptions and live data — explained once, without jargon.
The on-board diagnostics standard: one connector and one language of requests.
It exists so that any scanner can talk to any car: a 16-pin connector, a set of mandatory requests and a shared list of codes. Everything beyond that list each maker does its own way — which is why fault codes are read everywhere and a service reset is not.
The wire over which the car's units talk to each other and to you.
A car has not one unit but a dozen: engine, gearbox, ABS, airbags. They are linked by a shared bus, and the scanner connects to the same one. Two things follow: the adapter answers fast while the car answers slowly (it waits for the unit), and «no reply from the unit» means the question went out and nobody was there.
A sensor in the exhaust: it judges by leftover oxygen whether the mixture is rich or lean.
There is one before the catalytic converter (control) and one after it (monitor). By the first the unit adjusts fuelling, by the second it checks whether the converter still works.
The sensor that measures how much air enters the engine.
The unit decides how much fuel to inject from this reading. A dirty meter under-reads, the mixture goes lean, the driver presses harder, and consumption rises with the effort. That is why it is checked before expensive parts are replaced.
A part in the exhaust that burns off what the cylinders did not.
It works only when hot, so on a cold engine it does nothing — that is its design, not a fault. Its health shows in the rear oxygen sensor: with a healthy converter it stays flat, with a tired one it starts copying the front sensor's swings. It rarely dies on its own: misfires and a rich mixture kill it.
Returning part of the exhaust to the intake to lower combustion temperature.
It is needed at mid load; at idle and full throttle the unit does not ask for it. The valve clogs with soot, and then the command is there while the flow is not. A blanking plate removes the code and adds two problems: combustion temperature rises and the particulate filter clogs faster.
A protective state in which the unit deliberately limits the engine.
The car will not go past sixty, will not rev above three thousand, sometimes shuts off the turbo. This is not a fault in itself but a decision: the unit saw something that makes normal driving unsafe. A code is already stored at that moment — and it explains why.