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Error code reference

Denso Air Conditioner Error Code AL

Detection of unit stop signal from fire alarm system

Denso codesDenso air conditioner codes

Detection of unit stop signal from fire alarm system — input fire alarm signal received from fire alarm system.

First step from the service manual

Check fire alarm signal off. To reset: hold down SET TEMP UP and DOWN buttons simultaneously for 3 seconds.

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

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What AL means on a Denso air conditioner

The AL error code on a Denso air conditioner indicates that the unit has received a stop signal from a connected fire alarm system. This is an intentional safety interlock, not a mechanical or electrical fault within the air conditioner itself. When a fire alarm system triggers and sends its output signal to the AC unit's fire alarm input terminal, the Denso unit halts operation immediately to prevent the spread of smoke or combustion gases through the ductwork or refrigerant circuit area.

The signal originates from an external fire alarm control panel or detection device wired to the designated fire alarm signal input on the Denso unit. When this input receives the activation signal, the controller registers the AL code and shuts down the unit. Because the source of the interlock is external, the air conditioner itself may be fully functional. The key diagnostic step is to verify that the fire alarm signal has been cleared and that no active fire condition exists before attempting any reset procedure.

To reset the AL error after confirming the fire alarm signal is off, press and hold the SET TEMP UP and SET TEMP DOWN buttons simultaneously for 3 seconds. If the fire alarm signal remains active or the wiring between the fire alarm panel and the AC unit is faulted in a closed or triggered state, the AL code will persist or return immediately after the reset attempt. In that case, the fire alarm system itself or its interconnecting wiring to the AC unit must be inspected and cleared before normal operation can resume.

Source: manufacturer service documentation.

Common causes of AL

  1. 01

    Active fire alarm system trigger

    Wiring / connection

    The building's fire alarm system detected smoke or heat and sent a legitimate stop signal to the AC unit. Verify the alarm panel shows a clear status before resetting the air conditioner.

  2. 02

    Fire alarm system not reset after incident

    Wiring / connection

    Even after a fire alarm event is resolved, the alarm panel may remain in a triggered state, continuing to hold the AC stop signal active. The fire alarm panel must be fully reset by authorized personnel to clear this condition.

  3. 03

    Wiring fault causing false signal

    Wiring / connection

    A short circuit, damaged wire, or incorrect wiring connection at the fire alarm signal input terminal on the AC unit can cause the controller to read a continuous stop signal even when no alarm is active. Inspect the wiring at the fire alarm input terminals for damage, shorts, or miswiring.

  4. 04

    Fire alarm control panel malfunction

    Part replacement

    A fault within the fire alarm control panel itself can cause it to output a continuous stop signal to connected equipment. A licensed fire alarm technician should inspect the panel if no alarm condition is present but the signal persists.

  5. 05

    Incorrect integration wiring during installation

    Wiring / connection

    If the fire alarm interlock wiring was connected incorrectly during original installation, the AC unit may read the input as permanently triggered. Review the wiring configuration at the fire alarm input terminal block on the Denso unit.

See the test procedure for each cause

Frequently asked questions about AL

What does AL mean on a Denso air_conditioner?
The AL code means the Denso air conditioner has received a stop signal from a connected fire alarm system and has shut down as a safety measure. This is an intentional interlock function, not a unit malfunction. The air conditioner will remain off until the fire alarm signal is cleared and the unit is manually reset. Once the alarm condition is resolved, press and hold the SET TEMP UP and SET TEMP DOWN buttons at the same time for 3 seconds to reset the unit.
How do I clear the AL error code on my Denso air conditioner?
First, confirm that the fire alarm system has been fully cleared and is no longer sending a stop signal to the AC unit. Once the fire alarm signal is off, press and hold the SET TEMP UP and SET TEMP DOWN buttons simultaneously for 3 seconds to reset the AL code. If the code returns immediately after the reset, the fire alarm system or its wiring to the AC unit still requires attention.
Is the AL error on a Denso air conditioner a safety concern?
Yes, the AL code should always be taken seriously because it means a fire alarm system has signaled the air conditioner to stop. Before resetting the unit, you should confirm there is no active fire or smoke condition and that the fire alarm system has been properly cleared by authorized personnel. Never bypass or ignore this interlock.
Will resetting the AL code permanently fix the problem on a Denso air conditioner?
The reset will clear the AL code only if the underlying fire alarm signal has been deactivated. If the fire alarm panel is still in a triggered state, or if there is a wiring fault causing a false signal at the AC input terminal, the AL code will return after the reset. Persistent AL codes require investigation of the fire alarm system and its interconnecting wiring, not repeated resets of the air conditioner.
How much does it cost to fix an AL error on a Denso air conditioner?
If the AL code is caused by a legitimate fire alarm trigger that has since been cleared, the reset costs nothing beyond the technician time to perform it. If faulty wiring between the fire alarm panel and the AC unit is responsible, repair costs can range from around 75 to 250 dollars depending on the extent of the wiring work needed. If the fire alarm control panel itself requires service or replacement, costs will be higher and will depend on a licensed fire alarm contractor's assessment.

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