The by-pass connection is in the rear of the base unit. They are two parallel hoses attached by quick disconnects. You can find them on the side of the reservoir toward the bottom.
Error 053 is generated when high pressure is sensed on the cooling tip coolant line. The accessible checks you can make are ensuring a proper connection on the by-pass lines (remove them and reconnect them), ensuring proper connection of the attached IPL (remove and reconnect listening for 4 distinct clicks as the for 4 coolant lines connect), and replacing the filters.
You may also try changing the IPL. If the problem is resolved, then the offending IPL is clogged and will need to be serviced.
Senior Laser Engineer