You’re standing in front of your building’s HVAC controller, and a cryptic code like “U4” or “CH38” flashes on the screen. What now? Should your in-house team reset the unit, or is this a red-alert situation that needs a licensed technician immediately? If you're like many facility managers or building engineers, you’ve run into this frustrating moment, right when indoor comfort or system uptime matters most.
At Harold Brothers Mechanical, we’ve spent decades working with some of the most common commercial HVAC brands: Trane, Daikin, Mitsubishi, LG, and Fujitsu across hospitals, universities, industrial facilities, and office buildings throughout New England. Our licensed technicians deal with these systems every day, and we know firsthand how critical it is to understand which errors you can safely reset and which require professional help.
In this article, we’ll break down the most common HVAC error codes by brand, explain what they mean in plain English, and help you know exactly when your team can take action—and when to call in the pros. Let’s demystify those blinking codes and put the power of prevention and clarity in your hands.
No—error codes vary by brand, model, and controller type. Even within a single brand like Trane or Mitsubishi, codes may differ between system lines such as VRF, rooftop, or ductless split systems.
That’s why it’s important to know what model line you’re working with. The following sections focus on the most popular commercial units in New England and the error codes those units use.
Many modern commercial HVAC systems, especially those from Trane and Daikin, integrate directly with BAS interfaces to display error codes, schedule maintenance, and optimize performance. If you're considering a smarter facility setup, this guide on the cost and value of a Building Management System (BMS) explains how it can reduce energy waste and service calls.
Trane is one of the most widely used HVAC manufacturers in commercial buildings across the U.S., known for its reliability and integration with building automation systems (BAS). Their systems are common in schools, offices, and industrial facilities and are often paired with Tracer controls for smart diagnostics and performance monitoring.
Trane RTUs typically use 2–3 character alphanumeric codes, while Tracer systems may display codes through a Building Automation System.
May Be Handled by In-House Maintenance:
Requires a Licensed HVAC Technician:
On touchscreen panels, BAS terminals, or Voyager control boards.
Daikin is a global leader in HVAC technology and one of the pioneers of Variable Refrigerant Volume (VRV) systems. Their commercial equipment is known for energy efficiency, advanced controls, and seamless scalability in mid-sized to large commercial facilities. Ignoring HVAC error codes can lead to breakdowns that disrupt operations. If you're already facing downtime, here's how to keep your business running during HVAC failures.
Daikin’s VRV/VRF systems use two-character alphanumeric codes, such as U4, J6, or L5, often seen on wired controllers or BMS screens.
May Be Handled by In-House Maintenance:
Requires a Licensed HVAC Technician:
On wall-mounted Daikin controllers or BMS software via Intelligent Touch.
Mitsubishi Electric specializes in ductless and variable refrigerant flow (VRF) systems tailored for energy-efficient zone control. Their City Multi systems are a popular choice in offices, schools, and retrofits, especially where flexible zoning and quiet operation are priorities.
Mitsubishi uses 2-letter or alphanumeric codes like E6, U0, or P8, displayed on wall-mounted wired controllers.
May Be Handled by In-House Maintenance:
Requires a Licensed HVAC Technician:
On PAR or PAC controllers, some models support error history lookup.
LG’s Multi V systems bring cutting-edge inverter technology and centralized control to the commercial HVAC space. They’re often found in multi-tenant office buildings, hospitals, and retail centers where precise comfort and aesthetics matter.
LG uses CH (Check) codes or alphanumeric codes like CH38, CH05, or L6, depending on the controller type.
May Be Handled by In-House Maintenance:
Requires a Licensed HVAC Technician:
On AC Smart touch controllers, handheld diagnostic tools, or LED sequences.
Fujitsu’s Airstage systems are popular in light commercial and mixed-use buildings for their compact design and ease of installation. Their VRF systems offer strong performance in both ducted and ductless configurations, supported by advanced control options and service tools.
Fujitsu uses 2-digit numeric codes or LED blink codes, sometimes displayed through the remote or external diagnostic interface.
May Be Handled by In-House Maintenance:
Requires a Licensed HVAC Technician:
Via UTY wall controller or USB diagnostic tool.
Commercial HVAC systems are designed with advanced diagnostics to prevent small issues from becoming system-wide failures. But each manufacturer uses different error codes, and even within a single brand, these codes vary by model and controller. Knowing what they mean isn't just helpful—it’s critical.
Now that you have a clearer understanding of what those codes mean across Trane, Daikin, Mitsubishi, LG, and Fujitsu systems, you can make faster, smarter decisions when alerts pop up. You’ll know when it’s a simple filter check… and when it’s time to shut the system down and call in a certified tech.
Want to minimize downtime and avoid the chaos that can come with surprise HVAC issues? Learn how to keep your building running smoothly, even during major HVAC repairs, with practical tips from our experts. Discover how to stay operational when your HVAC system goes down.
At Harold Brothers Mechanical Contractors, we’re not just HVAC experts—we’re your partners in keeping commercial buildings running smoothly. Whether you're maintaining a campus of rooftop units or managing a multi-zone VRF system, our licensed professionals are here to help you keep error codes from turning into emergencies.