Generally speaking, the OBD system does not light the MIL lamp immediately after detecting a fault, but needs to be confirmed by a new diagnosis. Generally speaking, the MIL lamp is lit if a fault is detected twice or three times. A fault is called an accidental fault before lighting the MIL lamp. The fault that lights up the MIL lamp is called a confirmed fault.
Does that mean that the OBD system failed to detect fault if MIL does not light after starting? No, it isn't. There are two situations that may cause MIL is not lit when the OBD system found faults. One is accidental failure, the other is non emission related failure.
The MIL lights simply indicate faults that cause vehicle emissions which exceed the OBD limits set by the regulations. Even if the MIL light is not alarms, there may be other faults that are not related to emissions. For example, air conditioning line failure, clutch switch circuit (if any).
Many OBD systems use another fault lamp to indicate non-emission-related faults, often in the words EPC or SVS on the dashboard, but this fault lamp is not mandatory by law. Whether there is a second fault lamp or not, the fault-related information diagnosed by the OBD system is stored in the fault memory, which can be obtained by OBD-II code reader or diagnostic scan tool through the diagnostic connector on the car .