How to Troubleshoot Your HVAC’s Most Common Warning Lights

If your HVAC system is flashing a warning light, don’t panic—your system is trying to tell you something important. Modern heating and cooling systems are equipped with self-diagnostic technology that helps identify potential issues before they turn into major problems.

In this guide, we’ll walk you through the most common HVAC warning lights, what they might mean, and when it’s time to call in a professional.

Why HVAC Warning Lights Matter

Those blinking lights on your indoor unit, outdoor condenser, or thermostat aren't just there to be annoying. They're designed to give you real-time feedback on your system's performance.

Some issues may be minor and temporary—others could point to a problem that, if ignored, might damage your system or spike your energy bills.

Common HVAC Warning Lights & What They Mean

Here are the most frequent warning lights homeowners see, especially in Florida’s humid climate:

Blinking Green or Red Light on the Furnace Board

If you check the panel on your indoor HVAC unit and see a blinking light, it may correspond to a fault code. Each brand has a different flash sequence, but here’s what they often mean:

• Steady Green: System is functioning normally.
• Blinking Green: A minor fault (could be a dirty filter or thermostat miscommunication).
• Blinking Red: A more serious issue like overheating, flame rollout, or a sensor failure.

Troubleshoot:

• Replace the air filter.
• Reset the system by turning off the breaker for 30 seconds.
• Check your thermostat batteries and settings.

Call Perfect Air If: The red light persists after a reset, or your home isn’t heating/cooling properly.

2. Flashing Yellow Light on Outdoor AC Unit

This light often means the compressor is locked out due to an electrical or refrigerant issue. It may also indicate:

• Low refrigerant
• High head pressure (often due to a dirty condenser coil)
• Short cycling or overheating

Troubleshoot:

• Make sure the outdoor unit is free from debris and plants.
• Turn off the system for 10 minutes and restart.
• Check your thermostat for rapid on-off behavior.

Call Perfect Air If: The light keeps flashing or your system isn’t cooling.

3. Thermostat Error Codes

Smart thermostats like Nest, Ecobee, and Honeywell may flash symbols or error messages, such as:

• “No Power to Y1”: Your AC isn’t receiving a signal.
• “RH Wire Not Detected”: No power from the furnace board.
• Flashing Snowflake or Flame Icons: Delayed start due to a safety timer or issue with the compressor.

Troubleshoot:

• Check your home’s circuit breaker.
• Replace thermostat batteries.
• Inspect wiring if you’re comfortable.

Call Perfect Air If: You see wiring errors or feel uncomfortable opening your thermostat.

4. Blower Motor or Airflow Indicator Lights

Some systems have diagnostics tied to airflow sensors. If airflow is too low, the system may shut down or blink a code to protect components like the heat exchanger.

Common Causes:

• Dirty filter
• Blocked or closed vents
• Failing blower motor

Troubleshoot:

• Change the filter (even if it’s not due).
• Open all vents.
• Listen for unusual motor sounds.

Call Perfect Air If: The system won't stay on, or airflow is weak in all rooms.

Bonus Tip: Don’t Ignore Ice or Water Near Your HVAC

If you notice your indoor coil frozen or puddles around the unit, it could be tied to the same warning lights. Low refrigerant, clogged drains, or restricted airflow often go hand in hand with system alerts.

When to Call Perfect Air

While it’s great to know how to spot and reset minor issues, HVAC systems can be complex and potentially dangerous if misdiagnosed. You should call Perfect Air if:

• A warning light returns after a reset
• Your system doesn’t heat or cool properly
• Strange smells, noises, or water appear
• You’re unsure what the code means

At Perfect Air, we specialize in HVAC diagnostics, repairs, and maintenance in the Pensacola and Gulf Coast region. Our techs are trained to handle systems of all makes and models—so you can stay cool and safe.

Need Fast, Reliable Help?

Schedule a service call with Perfect Air today and let our expert team get your system back on track—before that blinking light becomes a full system shutdown.