Karcher k7.91 pressure washer /loss of pressure

Your pressure washer is either not fitted with a run/stop device in which case like mine you have to switch off within a few seconds of closing the trigger. Check your model features?
The humming noise is the motor stalled out against the pump because there is nowhere for the water to go. The shutoff is caused by the safety overload switching off after overheat.
Or, it has a pressure operated run/stop which is not working. Because water and electricity don't mix, I would suggest getting more specific help. You can download Karcher manuals /spares lists online from Karcher specialists but remember electricity is the silent killer.

The valve that admits more water to be compressed is stuck. It is likely the O-Rings on it need replacing. No water to the pump, no pressure!

The motor will stop when pressure is up to the designed pressure.

replace the fuel carburetor filter and, or, your fuel tank filter

Check for possible loose connections, its possible it could be leaking which would cause the loss of pressure.

Is you nozzle plugged? That is what this sounds like.

bob I had this happen to my Karcher, the pressure seals have blown. $Au200 for service and replacement from karcher dealer. tyhey would not sell the parts for a DFY

give me a more detailed explanation of how this happened and what you've tried

if the machine is blowing fuses then it could be the motor! if not then maby the start cupasitor has failed

Before you start it up:
1. Disconnect the high pressure hose 2. Connect the water supply and turn on 3. Make sure water is flowing through the pump
Start the motor, run for about 10 seconds and then turn off.
Turn off the water supply, connect the high pressure hose, turn water back on, start the motor and away you go.

411a when i turn on it gos for 1 min then i have too press it on and off 3or 4 times befor it works and it only stays on for 1 min.