Karcher 620m runs for e.g. A split second, sometimes 1 second, then trips a separate RCD. Is this a wiring fault in the motor? The on/off switch internals look ok. I have had it running for several seconds, but when out back together this fault returns.
Find someone who understands electricity and check each part fo rearth leakage. Whe n you fin dthe part consider replacing the part of if it would be cheaper to replace the Karcher.
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Try fixfault.com to fix general d.i.y.