Signal transmission error between the indoor unit and the outdoor unit.
Daikin U4 on the FTX-N service manual is a signal transmission error between the indoor and outdoor units. It is not A5. A5 is freeze-up protection in cooling or heating peak-cut, and A5 is the code whose checks start with the air path and the indoor filter.
Likely causes
- Power supply out of specification
- Wiring error or a broken connecting wire between the indoor and outdoor units (wire No. 3)
- Defective indoor or outdoor unit PCB
- Short circuit inside the outdoor fan motor winding
Safe checks for U4
- Write the code down, then switch off the outdoor disconnect and the breaker. Leave both cabinets closed
- Technicianthe manual checks supply voltage, the interconnecting wires, insulation on terminals 1-3 and 2-3, outdoor LED A, and whether the outdoor fan turns freely
Capacitors can hold a charge after the power is off. Safety terms.
When to call a pro
U4 is a communication fault, not a filter fault. Do not open the outdoor unit. Call a licensed HVAC technician.
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Parts & tools for this fix
Taken from this code's causes or steps. A search link opens Amazon results and does not confirm a fit. Check the model number in the manual.
Multimeter for appliance and HVAC diagnosis: guide
Technician step in this code: the manual checks supply voltage, the interconnecting wires, insulation on terminals 1-3 and 2-3, outdoor LED A, and whether the outdoor fan turns freely
This one is for a technician to test or replace, so we only explain it and do not link to buy it.