Daikin U4 Error Code – Malfunction of Transmission Between Indoor and Outdoor Unit

Air Conditioner · HIGH Severity

HIGH severity Air Conditioner Model: Daikin Inverter 18000 BTU

What This Error Means

Short circuit in transmission wiring, outdoor unit power supply OFF, system address mismatch or defective PCB

Diagnostic & Repair Steps

Check Communication Wire
Inspect the communication cable between indoor and outdoor unit. Look for cuts, pinches, or disconnections. 🔧 Flashlight

INSPECT ~10 min

Check Terminal Block Connections
Verify correct wire connections at both indoor and outdoor terminal blocks. Ensure wires are on correct terminals (L, N, 1, 2, 3 or S). 🔧 Screwdriver, multimeter ⚠️ Disconnect power before inspecting terminal connections

CHECK ~10 min

Test Communication Signal
Set multimeter to AC Volts. Measure between communication terminals while unit is powered. Should show ~12-24V AC pulsing signal. 🔧 Multimeter

TEST ~15 min

Check PCB for Damage
Inspect indoor and outdoor unit PCBs for burnt components, swollen capacitors, or visible physical damage. 🔧 Flashlight ⚠️ Discharge capacitors before handling PCB

INSPECT ~10 min

Replace Faulty PCB
If wiring is intact but no signal is present, replace the faulty PCB (usually indoor PCB first, then outdoor). 🔧 Screwdrivers, ESD wrist strap

REPLACE ~30 min

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