Why won't my aircon turn on?
A dead aircon breaks at one of four points: the supply stops before it reaches the board, the indoor and outdoor units lose their communication handshake, the board sits in a protective lockout from an earlier fault, or the board's own startup circuit has died. Four faults, one blank panel.
By Team Snowflake | Reviewed 30 May 2026
1. Power-supply path interruption
Nothing responds, anywhere. The supply path runs from the distribution board, through a dedicated wall isolator, down to the indoor terminal block, then across to the outdoor compressor. A break anywhere in that chain produces the same dead result. The isolator is the most common failure point. In humid HDB service yards the contacts corrode over years of switching on and off.
How to tell
With a broken supply path, there is no light, beep, fan movement, or flicker from the first second. Unlike a handshake fault, the display never wakes. Unlike protection lockout, there is no short start attempt. Unlike a board startup fault, voltage is not confirmed at the board yet.
- No indicator light at all when you press the remote.
- No acknowledgement beep.
- No sound or movement from either the indoor or outdoor unit.
How we confirm it
We trace power from the distribution board through the isolator to the terminal block, finding the point where voltage drops out. Then we restore or replace the failed point, most often the corroded isolator.
Do not approve a board or capacitor before the isolator and terminal block are tested. The isolator is a cheap common failure point.
2. Indoor-outdoor handshake failure
The display lights up, but the unit never starts cooling. Split systems need a live handshake between the indoor and outdoor boards before cooling begins. A corroded terminal, pinched wire, or partly failed outdoor board can break that link while standby power keeps the panel alive.
How to tell
The unit looks awake: the display lights and the remote beeps, but cooling never starts. Unlike a supply break, the panel is not dead. Unlike a lockout, there is no short start and clean cutoff. The indoor side is waiting for the outdoor-side handshake.
- Display lights and the remote beeps, but no cooling cycle starts.
- Response is on-and-off across repeated attempts.
- Outdoor unit stays silent while the indoor panel stays lit.
How we confirm it
We measure the communication wire voltage and check the sensor readings on both boards. Then we step through the handshake sequence to isolate where the link breaks. Once the break is found, we repair the corroded terminal or wire.
Do not start with a new remote before checking the communication wire and outdoor board. HDB installs more often fail at the comms terminal.
3. Control-board protection lockout
The unit tries to start, then stops itself. Control boards keep a fault history in memory that survives power-off. An earlier event can trigger this: an overcurrent surge, a sensor reading out of bounds, or a pressure trip. The board logs it and can hold a protective lockout. Cycling the isolator or pressing reset does not always clear it. The lock holds until the original trigger is confirmed safe.
How to tell
Protection lockout shows life, then cuts cleanly. The display responds, the unit tries to start, and it stops the same way each time. Unlike a supply break, power is present. Unlike handshake failure, it does attempt to start. A repeated short start points to stored protection.
- Display is on and responds, but startup never completes.
- A short startup attempt begins, then stops abruptly.
- The same clean cutoff repeats on every command.
How we confirm it
We read the board fault memory first. Then we clear the protection only if the trigger is safe and verify the original cause before restart.
Do not force repeated restarts to try to clear it yourself. Each forced attempt overwrites the fault data we need to read from memory. That hides the original trigger and turns a quick read into a guessing game. Stop after two attempts and let the diagnostic happen first.
4. Indoor board startup circuit failure
Voltage reaches the board, but the board never wakes up. Mains is confirmed at the terminal block, yet the display still stays blank. The fault sits inside the board's own startup circuit: the fuse, the transformer, or the components that stabilise the startup power rail. Any one of them can leave the panel dark even though full supply is sitting right at the connector.
How to tell
Here voltage reaches the board, but the display stays blank. Unlike protection lockout, there is no stored event or brief start. Unlike supply interruption, a meter confirms mains at the terminal block. The failure is inside the board startup circuit.
- Panel stays blank even after a stable power reset.
- A meter confirms mains voltage present at the terminal block.
- Other units in the same flat run normally.
How we confirm it
We confirm mains voltage at the board connector, then check the internal fuse and the transformer output. That isolates whether the board itself failed or just a single support component.
Do not replace the whole board before checking the internal fuse and transformer output. Voltage in with no low-voltage output can be a smaller support-component fault.
Related reading
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