Your Guide to Additional Fire Alarm Codes (Part 3)

By Andrew Erickson

July 16, 2023

Alarm codes and trouble codes are among the most commonly searched items when you're troubleshooting a fire alarm system. I've even heard from many of you who have called us (1-800-523-7232), emailed (, or used the online chat option on this website.

That's why I'm adding "Part 3" to complement my original articles on understanding fire alarm trouble codes and fire alarm codes. I'm happy to continue providing definitions like these so that you can keep moving forward with your project.

Now, to continue our discussion, check out this list of common alarm codes:

"Line Fault"

The "Line Fault" trouble code indicates an issue with the phone line or other communication lines used by the fire alarm system. This might be due to a cut line, a line short, or other communication interruptions. Communication faults should be addressed immediately to ensure the system can communicate with the central station or other monitoring services.

Increasingly, you're going to see fewer dialers and more internet-based or LAN-based systems. That increases the overall complexity, so you may start to see things like firewall issues or network outages triggering alarm codes like this one.

"Detector Sensitivity Out of Range"

The "Detector Sensitivity Out of Range" trouble code signifies that one or more of the fire alarm system's detectors are either too sensitive or not sensitive enough. This could lead to false alarms or, in the worst case, a failure to detect an actual fire.

Some sensors have settings that allow you to perform adjustments, while others have simply failed and require replacement.

Therefore, ultimately: Adjusting or replacing the affected detectors is necessary to resolve this trouble.

"Overcurrent Fault"

The "Overcurrent Fault" trouble code indicates that an excessive current is flowing through a part of the fire alarm system, which can result in damage to the components. The issue might be caused by a short circuit, faulty equipment, or incorrect wiring. The affected component should be located and repaired or replaced as needed.

Ironically, overcurrent conditions can actually cause a fire. You wouldn't want that to happen.

More likely, equipment damage will be expensive to diagnose and replace. It's also possible that damage won't be immediately noticed and cause trouble in the middle of the next emergency situation. That is a lurking nightmare you absolutely want to avoid.

"Relay Fault"

The "Relay Fault" trouble code signifies a problem with one or more of the system's relays. A malfunctioning relay can prevent the fire alarm system from functioning correctly, like failing to activate sounders or control other safety systems. The relay(s) should be checked and repaired or replaced if necessary.

This is an interesting alarm type, as relays themselves are a method of reporting alarms. In theory, there are a few possible formats for outputting a Relay Fault alarm. You can see a Relay Fault reported via another relay, on a fire panel's LCD display, or via protocol messages delivered via dialer, LAN, internet, radio, etc.

"Fire Door Trouble"

The "Fire Door Trouble" trouble code means that there's an issue with one or more of the fire doors in the building. This could be because a fire door is jammed or improperly closed. This may mean that it won't close correctly during a fire.

More likely, the door's sensor is simply malfunctioning. This must also be addressed promptly to avoid receiving bad information during an emergency.

Any trouble with fire doors, whether actual or due to a sensor problem, must be addressed promptly to maintain the building's fire safety.

"Heat Detector Fault"

The "Heat Detector Fault" trouble code signifies a problem with a heat detector in the system. This could be due to physical damage, electrical issues, or a faulty detector.

In most cases, your FACP (fire panel) is reporting that it cannot communicate with the detector. Heat detectors are so simple that, generally speaking, they're either online and working or entirely unavailable.

Any problematic heat detector should be repaired or replaced to ensure it can detect a fire.

"CO Detector Fault"

The "CO Detector Fault" trouble code indicates that there's an issue with a carbon monoxide detector in the system. This could be due to a low battery, physical damage, or sensor malfunction. Since carbon monoxide is a deadly gas, the detector should be repaired or replaced as soon as possible.

Similar to heat detection, CO detectors are similar device. The critical distinction here is that CO is odorless and causes drowsiness before death. That makes any flaw in CO detection a potentially silent killer.

"Silenced Alarm Active"

The "Silenced Alarm Active" trouble code is displayed when the fire alarm has been triggered, but the sounders have been silenced. This usually occurs when a fire department or trained personnel are addressing the situation, but the system is still in an alarm state.

If you know that silencing is expected during a test, drill, or actual fire response, you don't need to worry about this alarm. It is simply a reminder that unsilencing must occur before normal monitoring operation can resume.

"Module Fault"

The "Module Fault" trouble code means that there's an issue with one of the modules within the fire alarm system. Modules could include communication cards, power supply units, or other specific purpose units within the system. The specific module should be checked and repaired or replaced if necessary.

This is where NICET-certified technicians from an approved installer/distributor/reseller come into play. The right partner will able to both furnish and install replacement modules to resolve this type of alarm.

"Loop Isolation Fault"

The "Loop Isolation Fault" trouble code indicates a problem with the loop isolator in the fire alarm system. Loop isolators are designed to protect the rest of the system in case of a short circuit in one section.

This is similar to how bulkheads and compartmentalization work in a ship (and how fire doors work during a fire). If one section is compromised, the others can be sealed and remain unaffected. Many of us can recall a strand of Christmas lights entirely darkened by one bad bulb. The idea with loop isolation is to prevent this from happening.

An issue with a loop isolator could lead to a larger section of the system being affected by a fault, and hence, needs immediate attention.

Find Your Local Distributor/Installer (or Become One!)

At Digitize, we're a manufacturer of fire alarm monitoring systems. We can and will help you directly, and you can also get region-specific help from your local distributor/installer. Just give us a call. We'll connect you with a distributor in your region.

Would you like to become a Digitize distributor? Just give us a call and we'll work toward getting you set up.

Call us at 1-800-523-7232 or email

Andrew Erickson

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 17 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and...Read More