Mitsubishi Electric PURY-P72 Service Handbook page 256

[ IX Troubleshooting ]
1. Error Code
6834
MA controller signal reception error (Start bit detection error)
2. Error definition and error detection method
Communication between the MA remote controller and the indoor unit is not done properly.
No proper data has been received for 2 minutes.
3. Cause
1) Contact failure of the remote controller lines of MA remote controller or the indoor unit.
2) All the remote controllers are set to SUB.
3) Failure to meet wiring regulations
Wire length
Wire size
Number of remote controllers
Number of indoor units
4) The remote controller is removed after the installation without turning the power source off.
5) Noise interference on the remote controller transmission lines
6) Faulty circuit that is on the indoor board and performs transmission/ reception of the signal from the remote controller
7) Problems with the circuit on the remote controller that sends or receives the signals from the remote controller
4. Check method and remedy
1) Check for disconnected or loose transmission lines for the indoor units or MA remote controllers.
2) Confirm that the power is supplied to the main power source and the remote controller line.
3) Confirm that MA remote controller's capacity limit is not exceeded.
4) Check the sub/main setting of the MA remote controllers.One of them must be set to MAIN.
5) Diagnose the remote controller (described in the remote controller installation manual).
[OK]: no problems with the remote controller (check the wiring regulations)
[NO]: Replace the MA remote controller.
[6832, 6833, ERC]: Due to noise interference <Go to 6)>
6) Check wave shape/noise on MA remote controller line by following "IX [3] Investigation of Transmission Wave Shape/
Noise".(page 270)
7) When no problems are found with items 1) through 6), replace the indoor unit board or the MA remote controller.
The following status can be confirmed on LED1 and 2 on the indoor unit board.
If LED1 is lit, the main power source of the indoor unit is turned on
If LED2 is lit, the MA remote controller line is being powered.
HWE0803A - 247 - GB