Diagnosis
The automatic shut-off (ASO) valve is stuck open or failed. This valve stops production when the tank is full. A constant drain stream means it's not closing.
How to Fix It
First, try to free it by flushing. Turn off the tank valve. Let the system run for 5 minutes. The constant flow can sometimes dislodge a tiny debris particle. If it stops, turn the tank valve back on.
If That Didn't Work
The ASO valve is part of the filter housing assembly. It is not user-serviceable. You must replace the entire filter housing assembly (part #FH-600 for this model). Shut off feed water and power before replacement.
Heads Up
This wastes hundreds of gallons of water. If the flush doesn't work immediately, shut off the feed water valve under the sink until you can replace the part. Do not let it run unattended.