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only thing i cam think of is if u have air lock
maybe when cold remove sensor fill radiator with water
till it comes out of the sensor hole and while u r filling radiator
start to screw sensor back in
u might have done that already
there should not be thst much temp difference between the 2 positions
maybe u have too much thread tape and sensor has bad earth to block
try earthing sensor wire for split second to see if dash gauge goes to full
only for short period