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Error code reference

Denso Air Conditioner Error Code ALRM

Unit stop signal detected from fire alarm system

Denso codesDenso air conditioner codes

Unit stop signal detected from fire alarm system — external fire alarm input signal is active.

First step from the service manual

After input signal turns off, reset the system by pressing ON/OFF and HI/LO buttons simultaneously for 5 sec.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What ALRM means on a Denso air conditioner

The ALRM error code on a Denso air conditioner indicates that the unit has received an active stop signal through its external fire alarm input interface. This is a safety interlock feature built into the system: when a connected fire alarm panel or building fire detection system sends a signal to the Denso unit's dedicated fire alarm input terminal, the air conditioner shuts down immediately to prevent the HVAC system from circulating smoke or fanning flames through the ductwork. The unit will remain in this shutdown state for as long as the input signal is active.

The fire alarm input is a low-voltage signal circuit that interfaces directly between the building's fire alarm control panel and the air conditioner's control board. Under normal operating conditions, this circuit is in a defined idle state. When the fire alarm system activates, whether from a genuine fire event, a triggered detector, or a test sequence, it changes the state of this input, which the Denso control board interprets as a command to halt operation. The unit cannot be restarted by conventional means while this signal remains active.

To clear the ALRM code and restore normal operation, the fire alarm input signal must first be deactivated at the source. Once the signal has turned off, the system requires a manual reset: press and hold the ON/OFF and HI/LO buttons simultaneously for 5 seconds. If the error persists after the signal has cleared and the reset is performed, the wiring between the fire alarm panel and the air conditioner, or the control board's input circuit itself, should be inspected for faults such as short circuits or wiring errors that could be holding the input in an active state.

Source: manufacturer service documentation.

Common causes of ALRM

  1. 01

    Active fire alarm event or drill

    Wiring / connection

    The building's fire alarm system triggered legitimately, either from a detected fire, smoke, or a scheduled fire drill or test. The signal is intentional and the unit shut down as designed.

  2. 02

    Fire alarm system not fully reset after an event

    Wiring / connection

    The fire alarm panel was not fully cleared or reset after an alarm event, leaving the output signal to the air conditioner still active. Check the fire alarm control panel to confirm it has been completely reset.

  3. 03

    Short circuit or wiring fault in the fire alarm input circuit

    Wiring / connection

    A short circuit or miswired connection between the fire alarm panel and the air conditioner's external input terminals can hold the input signal in an active state, causing a persistent ALRM code even when no alarm is present. Inspect the wiring at both the terminal block on the unit and the fire alarm panel output.

  4. 04

    Faulty fire alarm panel output relay

    Part replacement

    The relay or output contact on the fire alarm panel that interfaces with the air conditioner may have failed in a closed or active state, continuously sending a stop signal to the unit. This would require inspection and repair of the fire alarm panel by a qualified fire systems technician.

  5. 05

    Control board input circuit fault

    Part replacement

    In rare cases, the input circuit on the Denso air conditioner's control board itself may have failed, causing it to misread the signal state as active even when the external wiring is correct. This would require control board diagnosis and potential replacement.

See the test procedure for each cause

Frequently asked questions about ALRM

What does ALRM mean on a Denso air_conditioner?
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.
How do I reset the ALRM error on my Denso air conditioner?
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.
Is the ALRM code on a Denso air conditioner a safety concern?
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.
Will resetting the Denso air conditioner clear the ALRM code permanently?
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.
How much does it cost to repair a Denso air conditioner showing the ALRM code?
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.

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