Naboo - How do you solve the error message “A05 boiler drain clogged alarm”?

A05 Drainage alarm warn that during boiler flush water was detected. 
As consequence unit deactivate the boiler and show the message. 

This message does not necessarily require the intervention of a technician; the causes of the failure can be external.

Description

During the boiler flushing cycle (drain wash or calout) drain sensor detect water after a timeout period. Issue can be due to scale deposit on drain probe to the drain valve or blocked drain.

Reset

Error reset automatically when a flush cycle is properly completed. 
Start the cycle from menu boiler/drain flush. 

Warning: a cycle done fron function test do not reset the error.

Checks

Perform following checks: 
- Check drain valve 
- Check drain probe.