Observe first. Guess less. Stay out of the dangerous parts.

• Confirm the thermostat is in COOL mode and set below room temperature.
• Look at the filter without opening equipment panels; replace it if visibly loaded and you know the correct size and direction.
• Confirm supply vents and return grilles are open and not blocked.
• From a safe distance, note whether the outdoor unit is running and whether ice is visible.

Indoor and outdoor temperatures, thermostat setting, airflow strength, when the problem began, and whether the outdoor unit starts.

• Confirm the thermostat is in HEAT mode and the setpoint is above room temperature.
• Check the filter and visible airflow paths.
• Note whether the indoor blower starts and whether the thermostat displays an error or low-battery message.
• If you know the system has a normal service switch, confirm it was not accidentally turned off.

System type if known, thermostat messages, whether the blower runs, any odor, and how long the system attempts to operate.

• Inspect the filter and confirm its airflow arrow points toward the equipment.
• Open supply registers and move furniture, rugs, or curtains away from returns.
• Compare airflow in several rooms and note which branches are weakest.
• Look for obvious disconnected or crushed accessible duct sections without entering unsafe attic areas.

Which rooms are affected, whether doors change the symptom, filter type, recent renovations, and whether the problem is new or longstanding.

• Turn cooling off if water is spreading or threatening a ceiling, floor, or electrical component.
• Protect the area below the leak and document where the water first appeared.
• Check whether the filter is badly restricted.
• Allow visible ice to thaw with cooling off; do not chip or heat it.

Location and amount of water, whether the drain has overflowed before, visible ice, filter condition, and recent runtime.

• Record the approximate length of each on and off cycle.
• Confirm the thermostat is not beside a heat source, open window, or supply register.
• Inspect the filter and visible airflow paths.
• If a breaker trips, leave the system off after one reset rather than resetting it repeatedly.

Cycle times, thermostat location, outdoor conditions, error messages, and whether the indoor and outdoor sections stop together.

• Identify whether it comes from a supply vent, indoor equipment, outdoor equipment, drain, or thermostat.
• Record a short audio or video from a safe distance.
• Note whether it happens at startup, shutdown, or throughout the cycle.
• Shut the system off for burning, electrical, gas, or strong chemical odors.

Sound description, exact location, timing, recent work, weather conditions, and any visible vibration.

• Check whether it uses replaceable batteries and install the specified type if needed.
• Confirm the display is not simply asleep or locked.
• Check the HVAC breaker once and note whether it has tripped.
• Look for an obvious nearby service switch that may have been turned off accidentally.

Thermostat model, display or error code, recent battery change, breaker status, and whether water is present near indoor equipment.

• Record indoor humidity with a reliable monitor if available.
• Confirm windows and exterior doors close fully and bathroom or kitchen exhaust fans are not left running continuously.
• Check the filter and note system cycle length.
• Look for visible drain overflow, sweating ducts, or condensation around supply registers.

Indoor humidity, outdoor weather, cycle length, affected rooms, thermostat settings, and whether the system recently changed.