The “Lost Connection” Problem
The U4 Error is the most common fault in Daikin Inverter ACs. Inverter technology relies on constant data exchange between the IDU (Indoor Unit) and ODU (Outdoor Unit) via the signal wire (usually Terminal 3).
The Logic: If the Indoor PCB sends a command (“Start Compressor at 50% speed”) and doesn’t get a “Roger that” reply from the Outdoor PCB within a few seconds, it triggers U4 and shuts down the system.
1 DIY Checks (Before Calling Us)
The 5-Minute Reset
Turn off your main MCB switch. Wait for 5 full minutes. Turn it back on. This resets the communication logic and often clears temporary glitches.
Check Terminal Block
If you can safely access the outdoor unit, check the wires marked 1, 2, and 3. Are they loose? Burnt? Loose neutral wires are a prime suspect.
Rat Bite on Cable
Look at the thick cable running from your indoor unit to the outdoor unit. If rats have chewed the signal wire, U4 is inevitable.
Atlas Aircon Repair Protocol
How we fix the U4 Fault:
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01.
Continuity Test We use a multimeter to check the 3-core communication wire. If there is a break in the middle, we replace the entire cable (no joints allowed).
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02.
PCB Fuse Check Outdoor PCBs have glass fuses. Voltage fluctuations often blow these fuses, killing the outdoor unit’s brain. Replacing the fuse restores power.
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03.
Communication Circuit Repair If wires and fuses are fine, the Optocoupler or Comms IC on the PCB is damaged. We perform component-level repair or replace the PCB.
Estimated Repair Cost (Atlas Aircon)