That's the dump off for the pressure regulator. Check your undercar to make sure no nozzles are clogged, you may need to change the regulator if not..
Its actually a dump off for the pressure safety, on the G5's they are used as a safety pressure relief, not a regulator, if there is water coming out of it something is wrong.
NJpdq is correct, check your uc nozzles for pluging, I have also seen stuck hp valves cause this to bypass water.
Ensure your control for the hp valves is operating correctly and switching the valve, and that the uc valve is opening when commanded to.