A short checklist can make the service visit more focused: note the equipment, symptoms, timing, thermostat behaviour and any visible error code.
Start with the actual system
Useful decisions come from the property, equipment and operating conditions rather than a generic answer. Record what the system is doing, when the issue occurs and what changed before the problem began.
Confirm the scope in writing
A written scope should identify the proposed work and the assumptions behind it. Ask which items are included, which conditions could change the work and what support applies after completion.