Daikin AC Error A0 — External Protection Device Active Fix

What it means

Daikin air conditioner A0 is the External Protection Device error documented on Daikin Global's own error codes page at daikin.com/products/ac/services/error_codes.
Daikin's exact description: 'the external protection device connected to terminal strip T1-T2 of the indoor unit is activated.'
This means an external safety device (typically a smoke detector, condensate overflow switch, or external interlock) wired to the T1-T2 terminals is signaling a fault state — the AC itself isn't broken, but something connected to it is asking the AC to stop.

Affected Models

Daikin commercial split-system air conditioners (FXA, FXM, FCQ, FBQ series)
Daikin VRV systems (indoor units wired with external protection)
Daikin Sky Air units with external interlock wiring
A0 appears only on units actually wired to an external protection device — residential units typically have nothing wired to T1-T2
Daikin's documentation: 'External protection device — the external protection device connected to T1-T2 of indoor unit is activated'

Common Causes

External safety device (condensate overflow switch, smoke detector) tripped
Wiring between T1-T2 and the external device damaged or loose
External device itself has failed (open circuit reads as 'activated')
Drain pan overflow float switch triggered (common on ceiling cassette installs)
Recent service work left the T1-T2 jumper missing on a unit that wasn't supposed to have an external device

How to Fix It

Identify what's wired to T1-T2.

Power off the indoor unit at the disconnect.
Open the electrical box and look at terminals T1 and T2 on the indoor unit's terminal strip.
If there are wires to an external device (drain float, smoke interlock, BMS), follow them to identify what device is connected.
If there's nothing wired and just a jumper, the jumper may have come loose — that also triggers A0.
Check a drain pan float switch first.

On ceiling cassettes and concealed ducted indoor units, the most common A0 cause is a drain pan float switch.
Pop open the drain pan access cover.
Check whether the pan is full of water — if so, the drain is clogged.
Clear the drain trap and vacuum the drain pan dry.
The float switch returns to its normal position and A0 clears.
Check the condensate drain line.

Trace the drain line from the indoor unit to the outdoor termination.
Look for kinks, sagging sections, or visible blockage.
Pour a litre of warm water mixed with a cup of vinegar into the drain trap to flush slime.
A properly draining condensate line keeps the float switch un-triggered and A0 stays cleared.
Test the external device with a multimeter.

If you can identify the external device (smoke detector, BMS interlock), disconnect it from T1-T2 and use a multimeter in continuity mode across its output contacts.
The contacts should be CLOSED (continuity) under normal conditions and only OPEN when the device alarms.
A device stuck OPEN triggers A0 even when there's no actual emergency.
Restore the jumper if no external device is intended.

Some Daikin residential models ship with a factory jumper across T1-T2.
If that jumper was removed during install or service and nothing else is wired there, A0 fires permanently.
Restore a small wire jumper across T1-T2 (only if no external device is intended) and A0 clears.
Schedule Daikin service if A0 persists.

If T1-T2 is correctly wired (either to a working external device or with the right jumper) and A0 still appears, the indoor PCB may misread the T1-T2 state.
That needs a Daikin-trained technician with a Daikin service tool to confirm and reset.
Schedule through daikin.com or your local Daikin service partner.

When to Call a Professional

Daikin's published guidance for A0 is to check the external protection device first, not the AC.
If the building's condensate drain overflow switch or smoke interlock has tripped, fix that — the AC will clear A0 automatically once the external device's contacts close again.
If nothing is supposed to be wired to T1-T2 and A0 still appears, a Daikin-trained installer needs to verify wiring and put the correct jumper back on T1-T2.

Frequently Asked Questions

What's the difference between A0 and A1 on a Daikin?

A0 is the external protection device error — something wired to T1-T2 of the indoor unit is signaling a fault.
A1 is an indoor unit PCB defect — the controller board inside the indoor unit has hit an internal self-test failure.
The two errors look similar on the wall remote, but the fix path is completely different: A0 you fix by checking external wiring and devices, A1 you fix by replacing the indoor PCB.
If you're getting both A0 and A1 alternately, the PCB is unreliable and needs replacement either way.