This is a genuine Karcher replacement part, it is sold individually.
When your pressure washer loses pressure, it probably means you have a bad spill valve.
Also known as an unloader valve.
This part allows water to recirculate in the pump when you're not spraying water and is responsible for most of the pressure-related problems with your pressure washer.
We do have an article and video on "How to Replace the Spill Valve on a Karcher Pressure Washer" on our site. Just go to the front page of our site and click on Repair Center.