Why is my aircon light flashing?
A flashing light is a fault signal, not the fault itself. The same blinking indicator covers a safety-circuit trip, a broken indoor-outdoor signal link, and a drifting sensor. The blink pattern and how long the unit runs before stopping tell the three apart.
By Team Snowflake | Reviewed 30 May 2026
1. Safety-trigger shutdown
The unit starts, cools for a short while, then stops and the light begins to flash. The control board has cut compressor power because a protection limit was crossed, most often refrigerant pressure climbing too high. That happens when the outdoor condenser cannot reject heat fast enough, or the circuit is overcharged. It will not restart until the fault clears after a cooldown or a deliberate reset.
How to tell
This path cools first and flashes later. Unlike a signal-link fault, cooling clearly starts before the stop. Unlike sensor drift, the blink count stays steady. Hot outdoor casing before each stop points to a real protection trip, not a random board fault.
- Unit starts and cools, then stops and flashes.
- The same blink count repeats on every restart attempt.
- Outdoor unit feels unusually hot before each stop.
How we confirm it
We check the outdoor startup path, including the capacitor and contactor, then measure pressure and compressor current. Compressor replacement is only suggested when the readings prove it.
Avoid quoting a new board before outdoor heat rejection, pressure, and compressor current are checked. The board may be reporting a real trip.
2. Indoor-outdoor signal-link fault
Cooling never settles. The unit may run for a moment then drop out, or refuse to start the outdoor side at all, while the light flashes one fixed pattern. The indoor and outdoor boards talk to each other over a signal wire run alongside the pipes. A corroded terminal, a pinched cable, or a failed driver on either board breaks that link, and the indoor board logs a communication fault.
How to tell
This path never settles into cooling. Unlike a safety trip, there is no strong cooling period before the flash. Unlike sensor drift, the blink pattern stays fixed each attempt. Corroded or browned communication terminals point to the signal link.
- The blink sequence looks identical on every attempt.
- System runs briefly, then drops out with no cooling.
- No build-up of heat or weakening airflow before the stop.
How we confirm it
We trace the communication wire between the indoor and outdoor boards. Then we check the terminal connections for corrosion and retest the full run after repair.
Avoid board replacement before the signal wire and terminals are checked. A cheaper terminal fault can look like a board failure.
3. Sensor or control instability
The fault feels random. Cooling works one run and quits the next, and the light may flash a coil-freeze or low-temperature code with no real freezing present. A thermistor that has drifted out of calibration feeds the board a reading colder than the coil actually is, so the protection trips on a false signal. Moisture in the sensor housing, physical damage, or age-related drift all cause this.
How to tell
This path changes between events. Unlike a safety trip, the blink speed, count, or sequence may shift. Unlike a communication fault, it may reset cleanly and return later with humidity or temperature change. That points to a drifting sensor reading.
- Blink speed, count, or sequence changes between events.
- Cooling behaves differently from one run to the next.
- The fault clears on reset, then returns with no clear trigger.
How we confirm it
We measure the thermistor resistance against spec and read the board output signals to isolate the drifting part before replacing anything.
Avoid swapping the control board before thermistor resistance is checked. A bad sensor can feed false data to a healthy board.
Related reading
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