U4/UF - The indoor and outdoor sections are not wired together correctly.
The official FNA-A/FNA-A9 installer guide groups U4 and UF together as an inter-unit branch wiring fault.
Grounded in the official Daikin FNA25~60A2VEB(9) installer reference guide error tables for commissioning and troubleshooting.
Severity
High
Display
Shown on the user interface during test run when the system detects an inter-unit communication or branch wiring problem.
Support first?
Often not
Models in range
FNA25A2VEB, FNA35A2VEB, FNA50A2VEB
What to do first
Start with these checks
- 1Turn the system off.
- 2Do not continue commissioning until wiring has been checked.
- 3Have an authorised installer verify branch wiring between indoor and outdoor units.
Likely causes
What usually sits behind this code
Inter-unit branch wiring is not correct
Indoor and outdoor sections are connected incorrectly
Commissioning setup or wiring sequence error
Contact support
Contact customer support if any of these apply
- !The code appears on first startup after installation.
- !There has been recent wiring or controller work.
- !Multiple indoor or outdoor components may have been cross-wired.
- - Daikin lists U4 and UF together for this family in the test-run error table.
- - This is a wiring and setup fault, not a washable-filter or routine maintenance reminder.
Related codes
Nearby codes in this family
Source notes
Source and scope
Source: Daikin Installer Reference Guide
Official FNA-A/FNA-A9 guidance links U4 and UF to incorrect inter-unit branch wiring.
Last reviewed: 2026-04-19
Daikin fault handling varies by indoor and outdoor unit combination, refrigerant type, and controller. Confirm the exact model pair and full fault history with an authorised Daikin technician before replacing parts.