Some faults are "latching" and others are "non-latching".
Latching is where it is a recognised fault in an emission critical (for example) system and it stays flagged regardless of what subsequently happens, then you have to go into the diagnostics with a tool to cancel/reset it.
Non-latching is when it is a minor issue seen by the ECU, something reading out of range etc, but if the system subsequently behaves acceptably the fault will no longer be flagged, i.e. it only flags as long as the perceived fault persists.
The category of faults is prescribed by regulation.
The OP needs to find out what the reason for the fault is with an OBD reader, as others say. It may be significant, it may not.