Most climate control trouble does not arrive all at once. The fan sounds tired, the air takes longer to warm up, or the cold setting stops feeling as cold as it did. These changes are easy to ignore, and they are easy to explain away. Treat them as a sign to look closer, because some of them touch visibility and safety.
The changes that show up first
Watch for airflow that is weak on one setting but strong on another. Note whether the fan speed seems to drop on its own, or whether the temperature drifts while you are driving. A slow warm-up on a cold morning, or cold air that never quite arrives, belongs on the list too. Write down what changed and when, because a repair shop will ask.
Sounds and smells worth noting
A clicking or cycling sound when the air conditioning starts can point to a problem in the cooling system. A sweet smell in the cabin needs prompt attention, because it can signal a coolant leak, which is a different system from the air controls and can affect the engine. Do not wait on a smell. Have it checked soon.
The fogged windshield is the urgent sign
If the defrost stops clearing the glass, treat it as a driving safety problem, not a comfort problem. Fog that will not clear makes it hard to see the road, and it can appear suddenly on a wet or cold day. Pull over somewhere safe when you cannot clear the glass, and get the system checked before you drive much farther.
Check the contract before you call the shop
If you have a service contract, compare the symptom against its parts list. Look for each named component, such as the blower motor, the compressor, or the heater parts, and read the exact wording. Do not assume a part is included because it sounds like part of the system. Ask the provider to confirm in writing how a particular component is handled before you authorize teardown work.
Shopping around for a contract
When you compare providers, ask how they treat climate control parts specifically. Ask whether any waiting period applies before a new contract pays for a repair, since a fresh contract with a waiting period may leave you covering an early failure yourself. This site's providers listed by waiting period page shows the published figures for named companies, so you can check before you buy.