that's probably the unloader valve all it might need is a quick dissasle to see whats gumming up the works , as all I was able to find for sale for that model here was a new gun and the soap injector for about sixty bucks each , you might have to look around in the Uk for one as that's were there most popular
The test / reset buttons are for a Ground Fault Interrupter built into the cord / washer. This device will help prevent deadly electric shock in the event of a malfunction - it is the same technology used in "GFI" electrical outlets located in bathrooms, outdoors, basements, etc.
You should have this serviced - or replaced before using.
There are no schematics available because the manufacturer doesn't sell parts for the gun. The spring usually goes right into the end of the hose, then the little bullet like valve follows. Did you know you can buy a new gun for about $22.00 Canadian?
1 water inlet filter clean ?? not limed or ??
2 nozzle clean.
3 pressure control valve properly screwed
Sometimes water hoses are provided with a slot at the hose connection with an internal water seal, which is the wrong way to the water tap instead of to the machine
I see this lots of times , the detergent tank will only mt in the low pressure mode of nozzle , this is the only way a siphion will develop inside the soap tank and then high pressure can be used later to wrinse the item off well using just water
Don't know where you are, but in my location here in Canada, the local LORDCO parts store has a large selection of pressure washer accessories.
I'd suggest that you try to find a similar equipment supply retailer.
I think there is hanging a by-pass (auto-stop) on lime after using it a long time can happen if after a long time before the first use the autostop or by-pass is a point where is placed mostly a microswitch that will standing in a open circuit state (activated and the spring is not strong enough to push the bypass back or i don know for shure if the material is ordinary steel or a phosphor bronze spring. but there is in much brands of pressure washers a lot of problems with that kind of technics if you use it not every 2 months 1 time then its giving problems the old models have a bypass only that the motor can run in non-load mode have the most pressure washers no problems, but there will be the problem with autostop / microswitch. https://www.youtube.com/watch?v=bm5XoFrAHyE
A bypass is by most pressure washers a plastic frame where is a microswitch inside look at this maybe you have luck if the spring is in good condition and only lime because the bypass have a broken spring most times the parts are expensive and in a complete cuff kit only , i am not a Karcher specialist only Kraenzle and Efco so i do not know how the service parts will going in your Country. succes