Daikin VRVII Manual page 294

Troubleshooting by Indication on the Remote Controller
Remote Controller Display
U4
Applicable Models
All indoor models
VRVIII-S and III-Q Series
Method of Error Detection
The error is generated when the micro-computer detects
that the transmission between the indoor and outdoor
unit is not normal.
Error Decision Conditions
When transmission is not carried out normally for a
certain amount of time
Supposed Causes
Indoor to outdoor, outdoor to outdoor transmission
wiring F1, F2 disconnection, short circuit or wrong
wiring
Outdoor unit power supply is OFF
System address does not match
Defective outdoor unit main PCB
Defective indoor unit PCB
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