Daikin Altherma HT - U4 Malfunction of Transmission between Indoor Unit and Outdoor Unit

Troubleshooting by Indication on the Remote Controller ESIE09-08B
200 Troubleshooting
3.56 “U4 ” Malfunction of Transmission between Indoor Unit
and Outdoor Unit
Remote Control
Display
U4
U4
subcode 0
Applicable
Models
AII indoor unit models
ER*Q 011~016*
Method of
Malfunction
Detection
Microcomputer checks if transmission between indoor and outdoor unit is normal.
Malfunction
Decision
Conditions
When transmission is not carried out normally for a certain amount of time
Supposed
Causes
 Indoor to outdoor transmission wiring F1, F2 disconnection, short circuit or wrong wiring
 Outdoor unit power supply is OFF
 Defect of outdoor unit PC board
 Defect of indoor unit PC board (A1P)