A warning light is the car asking for a decision. It does not say what is wrong, and it does not say who should act. In most households the light comes on, everyone assumes someone else will deal with it, and the car runs for weeks with the problem unresolved. Before you sign any ownership or service paperwork, settle who owns each step after a light comes on.
Read what the light means in the manual
Every warning light has a meaning set by the manufacturer, and the owner's manual is where that meaning is written down. Find the section that covers the dashboard indicators and read what each light calls for. Many manuals treat a steady light and a flashing light differently, and some lights ask you to stop driving at once. Keep a copy of that section in the glove box or on a phone, so the meaning is available when the light is on and nobody is sure what it is.
Name who reads the light
Decide who is responsible for noticing the light and reading the meaning. In a household with more than one driver, the default is that nobody is responsible, so write the job down. The person who reads the light should check the manual, decide whether the car can be driven, and tell the next person in the chain the same day. A light that stays on for days without a note is a light nobody owns.
Name who books the visit and who approves it
Once the light is read, someone has to call a shop, describe the symptom, and book the visit. Decide whether that person is the driver, the household's record keeper, or the owner of the car. Then decide who approves any work over the diagnosis. If the shop calls for approval and nobody picks up, the visit slips. Put the approval contact and the approval limit on the same sheet as the service record.
Do not clear the light to hide it
A cleared light is not a fixed car. Clearing a code or resetting an indicator without a repair erases the signal and leaves the cause in place. If a light goes off on its own, write down the date, the odometer reading, and what the light was doing, so the pattern can be shown to a mechanic later. A light that comes and goes is still information.
Write the entry down
Every light that comes on should produce a written entry: what the light showed, what the manual says it means, who read it, who booked the visit, what the shop found, and what was done. Keep those entries with the rest of the car's service paper. When you come to sell, transfer, or claim on any service paperwork, the entries are the proof that someone was keeping watch. Without them, the car's history is a set of guesses.