The unloader valve, which either increases or reduces the pressure, may have jammed stuck if machine has been idle for a while.
Try turning the valve knob either way to see if it makes any difference. If it doesn't, valve is the likely offender. If you are capable, using a spanner/wrench, to remove the valve which can then be cleaned.
However, recently, I took my valve off dissembled it and cleaned it, but alas, the valve still didn't work properly, so I ended up replacing with a new one.
Not that particularly costly and can be bought complete and ready to fit to machine.
Cut and Paste this on your browser.
http://www.espares.co.uk/part/pressure-washers/karcher/p/1315/877/0/0/553828/karcher-pressure-washer-u.html
Davymac, July 2009