Mitsubishi Electric PQHY-P72ZLMU-A Service Handbook page 270

[ IX Troubleshooting ]
1. Error Code
4121
Function setting error
2. Error source, cause, check method and remedy
Error source
Heat source (1) Dip switch setting error on the control board
unit
(2) Connector connection error on the control
board
(3) Control board failure
HWE15150
Cause
- 261 -
Check method and remedy
Check the SW6-1 setting on the control board.
Check that nothing is connected to the connector
CNAF on the control board.
Replace the control board if no problems are
found with the two items above.
GB