Karcher 411a motor stalls instead of switching off?

I've just replaced the bypass valve in my 411a machine and the pressure is now back to normal, but whan the trigger is released, the motor stalls instead of being switched off. The switch that should be turned off when the bypass valve opens seems ok. As a temporary procedure, to avoid burning out the motor, I could tape the trigger to keep it open, and use the mains switch to stop and start. Any suggestions what to do?