Mitsubishi Electric HP120TNU-A Service Handbook page 227

[7-8 Error Code Definitions and Solutions: Codes [6000 - 6999] ]
7-8-23 Error Code [6842]
1. Error code definition
Indoor-outdoor communication: Transmission error
2. Error definition and error detection method
Indoor/outdoor unit communication error (Transmitting error)
Abnormal if "1" receiving is detected 30 times continuously though indoor controller board has transmitted "0".
3. Cause, check method and remedy
(1) Defective transmitting receiving circuit of indoor controller board
(2) Noise has entered into power supply.
(3) Noise has entered into outdoor control wire.
Note: Refer also to the Service Handbook for the indoor units.
64
- chapter 7
Cause Check method and remedy
Turn the power off, and on again to check. If
abnormality generates again, replace indoor
controller board.
HWE18220_GB