Why Won't My Aircon Turn On?
A dead aircon usually points to a broken supply, a failed indoor-outdoor handshake, a protective lockout in board memory, or a failed startup circuit on the board itself. Where the chain breaks decides what gets repaired.
1. Power-Supply Path Interruption
How This Works
The supply path runs from the distribution board through a dedicated wall isolator, down to the indoor unit's terminal block, then across to the outdoor compressor. A break anywhere in this chain produces the same symptom: nothing responds. The isolator is the most common failure point, especially in humid HDB service yards where contacts corrode over years of thermal cycling.
How To Tell
A complete absence of indicator light, beep response, and startup sound from either unit is the clearest sign the supply has broken before reaching the board. Unlike a command-link failure, the indoor display is not lit. Unlike a control-board lockout, there is no brief startup attempt.
- No indicator light at all.
- No startup beep response.
- No indoor or outdoor startup sound.
How We'd Confirm It
We trace power from the DB board through the isolator to the terminal block. That tells us where the supply drops out. Then we restore or replace the failed point.
Do not replace the board or the capacitor before the supply path is checked. The dedicated isolator is the cheapest first check and it fails more often than the board itself.
2. Indoor-Outdoor Handshake Failure
How This Works
Modern split systems require a continuous handshake between the indoor and outdoor PCBs across a dedicated two-core communication wire. The indoor unit cannot begin a cooling cycle without this handshake. A corroded terminal, a pinched wire from an earlier install, or a partially failed outdoor PCB can break it without removing standby power.
How To Tell
The diagnostic signal is that standby power exists. The display lights up and the remote produces an acknowledgement beep, but no cooling cycle initiates. Unlike a power-path interruption, the indoor side is alive. Unlike a control-board lockout, there is no brief startup attempt that cuts off.
- Standby indication exists but no cooling cycle starts.
- Response is on-and-off across attempts.
- Behaviour changes without a stable pattern.
How We'd Confirm It
We check the communication wire voltage, sensor readings, and indoor-outdoor handshake sequence to isolate the break point.
Do not replace the remote before the communication wire and outdoor PCB are checked. A failed remote produces the same surface symptom but is far rarer than a corroded comms terminal in HDB installations.
3. Control-Board Protection Lockout
How This Works
Aircon control boards store fault history in memory that survives power-off. When a fault event occurs (an overcurrent surge, a sensor reading out of bounds, or a refrigerant pressure trip), the board logs it and can enter a protective lockout. Cycling the isolator or pressing reset does not always clear it; the lockout holds until the original trigger condition is confirmed safe.
How To Tell
The unit shows signs of life. The display is on and responds to commands, but every startup attempt ends with an abrupt stop. Unlike a command-link failure, the board is actually attempting to start. Unlike a power-path interruption, standby power is present. The brief startup that cuts off cleanly is the giveaway.
- Unit appears alive but startup does not complete.
- Short startup attempt then immediate stop.
- No sustained cycle despite repeated commands.
How We'd Confirm It
We read the board fault memory, clear the protection lock if safe, and verify the original trigger is resolved before allowing normal restart.
Do not force repeated restarts. Each one clears the fault data we need to read from memory, making the original trigger harder to find. Stop after two attempts and let the diagnostic happen.
4. Indoor Board Startup Circuit Failure
How This Works
When supply voltage is present at the terminal block but the board still fails to initialise, the fault is usually in the board's own startup circuit. That means the fuse, the transformer, or the capacitors that stabilise the startup power rail. Any of these can leave the display blank even though supply reaches the board.
How To Tell
Confirmed mains voltage at the terminal block is the prerequisite. With supply present, a blank display means the board's internal startup circuit has failed to convert mains into the regulated DC it needs. Unlike a protection lock, there is no fault event preceding the failure. Unlike a supply-path problem, a meter shows voltage present.
- Blank panel remains even after a stable power reset.
- No normal startup response from indoor unit.
- Other units in the same home may run normally.
How We'd Confirm It
First we confirm mains voltage at the board connector. If voltage is present, we check the internal fuse and transformer output. That isolates whether the board failed or just a support component.
Do not replace the whole board before the fuse and transformer outputs are checked. A blown internal fuse is the cheapest explanation and is often missed.
Related Reading
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