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Error Code
A3 means the unit has detected water where it should not be. A float switch in the drain pan trips when the water level rises, and the system shuts down to stop the ceiling or the wall cavity getting wet. The code is a flood warning rather than a refrigeration fault. In most cases the cause is a blocked drain line, a drain pump that has stopped, or an installation that never had enough fall on the pipe.
Dust, biofilm and insect nests build up in the drain line over a season, and the narrowest point on the run blocks first. Once water backs up to the float switch, A3 trips. It is the most common cause by a wide margin and also the cheapest to fix. A wet vac on the drain outlet clears most of them in minutes.
In cooling mode the drain line can freeze where it runs through a cold space or sits against an uninsulated surface. Ice blocks the pipe just as effectively as dirt and then melts into a puddle once the unit stops. Insulating the exposed section fixes it. If the pipe freezes repeatedly, the indoor coil needs looking at as well.
Where the installation has no gravity fall, a small pump lifts the condensate away. Those pumps have a float and an impeller, and both can jam with slime or scale. You can usually hear whether the pump is running when the unit starts. A pump that is silent but has power is failing, and it is a straightforward swap.
A3 on a system that has never worked properly points at the install. A drain line with no fall, a missing trap, a pipe running uphill or a pump fitted where gravity would have worked will all cause repeat A3 codes. This one is worth documenting, because the fix is re-running the drain rather than cleaning it again.
The drain is traced from the indoor pan to its outlet, and the run is checked for kinks, low spots where water can sit and any section that has been crushed behind a wall. The outlet is then cleared with a wet vac or low pressure water. Most A3 codes end here with no parts required.
Once the blockage is located it is flushed through until water runs freely and the pan empties completely. Pulling the float switch out and cleaning it at the same time avoids a repeat call, because slime on the float is what makes them stick. The pan is then dried and the unit run to confirm the drain keeps up.
On pumped installations the pump is tested on its own: power at the terminals, float moving freely, and water actually lifting away while the unit runs. A pump that hums but moves no water has a jammed impeller. One that does nothing has a failed motor or a float stuck down. Both are a swap rather than a repair.
If the drain has been cleaned and the pump works but A3 returns, the fall and the routing are re-checked against what the unit needs. That means a continuous downward slope, a trap where the manual calls for one, and insulation on any cold section. Correcting the pipework is the only permanent fix.
