What to Do When Your Commercial HVAC System Shows an Error Message
May 7th, 2025
3 min read
By Chris Cason
You walk into your facility and the HVAC controller is flashing a warning: "Fan Failure," "Sensor Error," or worse, just "System Fault." Whether you manage an office building, school, hospital, or warehouse, these alerts can lead to uncomfortable conditions and operational headaches — fast.
At Harold Brothers Mechanical Contractors, we’ve serviced thousands of commercial HVAC systems across New England. While specific error codes differ by brand (like Daikin, Trane, Carrier, and Mitsubishi), the underlying causes are nearly always the same.
This article explains the most common HVAC error messages you’ll see in commercial systems, what they actually mean, what you can check yourself, and when to call in your service team or electrician.
Common Commercial HVAC Error Messages and What They Really Mean
Commercial HVAC systems are built to protect themselves — and your building — by detecting problems before they cause major failures. When something goes wrong, these systems don't always explain it in plain language. Instead, you'll see warnings like “Fan Error” or “System Lockout.”
The good news? While the terminology may vary from one brand to another, most of these messages point to a handful of common underlying issues. Here's what those alerts actually mean and what you should do about them.
1. “Communication Error” / “Lost Signal” / “Indoor-Outdoor Fault”
What it means:
Your HVAC system's indoor units, outdoor units, and controllers aren’t communicating. Most modern systems rely on digital communication to sync operations. When that link fails, the system may shut down for safety.
Behind the scenes:
- Control wiring may be damaged or disconnected
- Power outage or surge may have scrambled communication
- A circuit board may be failing
What you can do:
Reset the system at the main disconnect or breaker. If it returns to normal, great. If not, it may be a deeper wiring or control issue.
When to Call Your Commercial HVAC Service Company:
If the error returns or if other equipment in your building is also acting up, call both your HVAC company and your facility’s electrician. Voltage drops, surges, or grounding problems could be affecting communication wiring.
2. “Sensor Error” / “Thermistor Fault” / “Temperature Out of Range”
What it means:
Your system is getting incorrect or missing data from one of its temperature or pressure sensors. This can lead to shutdowns or erratic behavior.
Behind the scenes:
- Loose, corroded, or shorted sensors
- Poor airflow is causing false readings
- Sensor positioning errors
What you can do:
Replace dirty filters and make sure vents are open and unobstructed.
When to Call Your Commercial HVAC Service Company:
If the error continues or the system short cycles. These sensors are sensitive and require diagnostic tools to test properly.
3. “High Pressure Lockout” / “Compressor Overload” / “System Protection Active”
What it means:
The unit has detected unsafe pressure or temperature levels and is protecting the compressor by shutting down.
Behind the scenes:
- Dirty condenser coils
- Blocked airflow
- Overcharged refrigerant
What you can do:
Clear outdoor units of leaves, dirt, or snow. Replace filters. Restore airflow.
When to Call Your Commercial HVAC Service Company:
If the error returns after a reset or if you hear humming, clicking, or overheating. Compressor protection is not something to ignore.
4. “Fan Failure” / “Blower Error” / “Airflow Malfunction”
What it means:
The system has detected a fan that isn’t turning on, is turning too slowly, or is obstructed. This disrupts airflow and can trigger freeze-ups or overheating.
Behind the scenes:
- Fan motor failure
- Relay or board issue
- Physical obstructions
What you can do:
Check for visible blockages around the outdoor unit. Make sure return and supply vents indoors are unobstructed.
When to Call Your Commercial HVAC Service Company:
If you hear strange noises, feel little or no airflow, or the issue repeats after reset.
5. “Low Pressure Warning” / “Refrigerant Loss Detected” / “Cooling Inactive”
What it means:
Your HVAC system doesn’t have enough refrigerant pressure to run safely.
Behind the scenes:
- Refrigerant leak
- Compressor or valve malfunction
- Failing pressure sensor
What you can do:
Nothing safely. Refrigerant work requires EPA certification.
When to Call Your Commercial HVAC Service Company:
Immediately. Running low on refrigerant can burn out your compressor and lead to expensive repairs.
6. “Voltage Fault” / “Inverter Error” / “Power Supply Issue”
What it means:
The system is detecting unstable voltage or power that is out of range for safe operation.
Behind the scenes:
- Building power issues
- Inverter board failure
- Phase imbalance in 3-phase wiring
What you can do:
Document the error. Check if lights or other building systems are also acting irregularly.
When to Call Your Commercial HVAC Service Company:
If the issue repeats, or if your system won’t restart after a reset, you should also contact your facility’s electrician to evaluate building voltage, phase balance, and grounding.
7. “System Lockout” / “Hard Fault” / Blinking Lights Without Message
What it means:
Your HVAC system has shut itself down completely after multiple or critical faults.
Behind the scenes:
- Several system errors that were not resolved
- Safety thresholds exceeded
- Internal software or hardware issue
What you can do:
Try a full power cycle and monitor the system. Document anything you see.
When to Call Your Commercial HVAC Service Company:
If it doesn’t restart or if lockouts are happening often. These require a full diagnostic check.
Don’t Let Error Messages Take Down Your Building
Error messages aren’t just technical glitches — they’re safety alerts designed to stop system damage and protect performance.
Understanding what the messages mean gives you the power to respond quickly and avoid costly downtime.
If your system is showing any of these alerts, and you would like to prevent downtime, read our guide to what HVAC parts you should always have on hand.
Harold Brothers Mechanical Contractors is your go-to commercial HVAC service team in New England. We diagnose and repair systems quickly, thoroughly, and with minimal disruption to your business.
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