CH and HW problem

My system has gone bananas. I have a Potterton Kingfisher 2 boiler, which fires up for about a minute, with higher than normal flue gas teperatures. The pump does not run unless I push the motorised valve (2 port) round with a screwdriver to operate the microswitch. Can anyone tell me where to start checking for the fault? When the pump is running it makes a sound like the water is cavitating within if that gives any clues.
Thanks
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Did you see the replies on your previous thread?
Before continuing, we need to know that all bleed valves are bled and that the system is full of water. If it is gravity fed, the header tank should be half full. If it is sealed, the pressure gauge should read at least 1 bar. Is this the case?
When was the system last drained and refilled? Was inhibitor added?
Christian.
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