We have a Karcher W2 windo vac. It stops after a few seconds. If you hold the blades open it will run. Can you say why?

How old is it? This can happen when the battery no longer holds much charge.

Spark plug?

There is a pressure switch that turns off the pump when it gets to a certain pressure. It sounds like that switch is sticking. The contacts should be closed when you first turn on the pump. If you hold open the trigger when you first turn on the pump and the pump stays on then that pressure switch is most likely the problem. If you turn on the pump with the trigger open and the pump runs for a few seconds and then turns off check for a blocked nozzle.

Replace the switch! It shut down when battery get depleted. and then when you start it again the cycle is shorter because the batery regenerate but not as previous cycle.

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 grease inside the foot controller is gone or dried up. Open it, clean it, and apply a little bit of new grease, Vaseline is fine. I shouldn't have to tell you to unplug it from the wall first.. should I?

hose is stopped up you have to plug the ends up when not in use mud dobbers and other bugs like to get inside had same problem before

Sounds like your unloader valve is sticking. It needs to be removed and lubed with lithium grease.

It sounds like there may be a pressure leak somewhere in the system. The machine cuts off when there is no flow of water past it. If it is leaking then there is intermittent flow. Which would make the machine run then stop continuously.

Probably your battery is bad. either charge or replace that and then learn the windows again. (have you got your radio code before removing battery?) to learn the windows do them one at a time with door closed. hold the down button you hear a clck a few seconds after it opens fully now hold the up/close button and again wait for the click. easiest is to hold the button pressed for 10 secs after the windo has fully traveled down then send it up and again 10 secs after it stops moving.