Service Manual

SiUS281811E Troubleshooting by Error Codes
Part 6 Service Diagnosis 249
4.50 Transmission Error between Indoor Units and Outdoor
Units
Error Code U4
Applicable
Models
All indoor and outdoor units
Method of Error
Detection
Microcomputer checks if transmission between indoor and outdoor units is normal.
Error 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
 System address does not match
 Defective indoor unit PCB
 Defective outdoor main PCB
Troubleshooting
Be sure to turn off the power switch before connecting or disconnecting
connectors, or parts may be damaged.
Caution
YES
NO
All indoor
unit remote controllers
of the same refrigerant system
display
U4
.
Are the
indoor-outdoor unit
transmission wirings
normal?
Has the
indoor or outdoor
unit PCB been
replaced, or has the
indoor-outdoor unit
transmission wiring
been modified?
NO
YES
Replace the indoor unit PCB
indicated by the error code
U4.
Fix the indoor-outdoor
unit transmission wiring.
NO
YES
Reset the power supply.
Press and hold the RESET
(BS5) button on the master
outdoor unit PCB for 5 seconds.
∗ The unit will not operate for
up to 12 minutes.
A
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