Engine fault codes essentially fall into two categories.

1. Electrical circuit faults. This is where a component or wiring is open or short circuit. The ECM is therefore unable to receive data from it.

2. Mechanical faults that are having a direct effect on engine emissions. These are more difficult to identify. The ECM will set a fault code or codes based on where the fault is being reported from. Please note this DOES NOT necessarily mean that a sensor or switch is faulty. The ECM cannot differentiate from a sensor being faulty (unless it is open or short circuit) from a sensor giving a wrong reading due to a mechanical fault.

The ECM (Engine Control Module) is a programmed computer. It cannot think, apply logic or reason. It does EXACTLY what it is told, nothing more and nothing less. It will only see something as a fault if it's receiving data outside of what it programmed to receive. So even if you can detect a fault with your car doesn't mean necessarily that the ECM can. This is where people commonly go wrong. No fault codes doesn't necessarily mean no faults, or vice-versa.