Denso Air Conditioner Error Code ALRM

The ALRM code means the Denso air conditioner has detected an active stop signal from a connected external fire alarm system. The unit shuts down automatically as a safety measure to prevent air circulation during a fire event. The code will remain displayed as long as the fire alarm input signal is active. Once the signal clears, the unit must be manually reset to resume operation.

Yes, the ALRM code is directly tied to your building's fire safety system and should be taken seriously. Do not attempt to bypass or override the fire alarm input circuit to force the air conditioner to run. If the code appeared without a known fire alarm event or drill, investigate the fire alarm panel and the wiring between the panel and the unit to identify the cause before resetting.

In many cases there is no repair cost at all: if the ALRM code was triggered by a legitimate fire alarm event or drill, simply resetting the system after the alarm clears is all that is needed. If a wiring fault is involved, labor costs for diagnosis and rewiring typically range from $75 to $200 depending on accessibility and local rates. If the air conditioner's control board has failed, replacement board costs can range from $150 to $500 or more for parts, plus labor.

First, confirm that the fire alarm input signal has been fully deactivated, meaning the fire alarm panel has been cleared and reset. Once the signal is off, press and hold the ON/OFF and HI/LO buttons simultaneously for 5 seconds to reset the air conditioner. If the ALRM code returns immediately after resetting, the fire alarm system may still be active, or there may be a wiring fault holding the input signal in an active state.

The reset will clear the ALRM code only if the underlying fire alarm input signal has been turned off first. If the signal is still active, the unit will not operate and the code will persist. If the cause was a one-time alarm event that has since been cleared, the reset should restore normal operation. If the code returns repeatedly without a corresponding alarm event, a wiring fault or component failure should be investigated.