A unit that has learned to work around its own fault can keep running for a long time while that fault gets worse. The workaround is usually a small habit, and because it works, it stops feeling like a problem. That habit is often the earliest record you have.
Spot the habits
Look for things you do that the unit was not designed to need. Examples include running a cycle again, holding a door shut with a wedge, adding water by hand, turning a dial past its marking, leaving a fan on to clear the air, emptying a tray more often than the manual suggests, or moving food away from a shelf that runs warm. Each habit is a sign that the unit is behaving differently from what you expect. Ask yourself when you started the habit and whether the start lines up with a change, such as a move, a power outage, a new pattern of use, or a repair.
Write the habit down before you call anyone
Keep a short log with three parts: what you do, when you started, and what the unit does when you do not do it. The log is not a claim on its own, but it gives a technician something specific to test and gives you a fixed reference if the symptom changes. Note whether the workaround is needed every time or only under certain conditions. A workaround that depends on weather, load, or season points toward a different cause than one needed on every use.
Read what the paper says about normal operation
Your owner's manual usually describes how the unit should behave under normal use. Compare your habit with that description. Then read your contract's wording on maintenance, notice, and wear, since many contracts ask you to report a fault within a set time or to keep the unit maintained in a particular way. This guide does not read any particular contract, so check your own paper for those clauses. If the paper requires notice, your log also shows when you first noticed the problem.
Questions to ask the technician
Ask whether the symptom you are working around falls within the range of normal operation for this model. Ask what the workaround is hiding, and whether the fault is likely to stop or to progress. Ask for the finding in writing, with the component named. Keep that answer with your log. If the answer is that the unit is operating normally, the log still shows what you did, and you can compare it against the next visit.