coolant temp 417

[slowpoke]

truck idling cold then heard fan come on, engine warn- coolant reading 417 deg. was running only about 5 minutes. shut it down unplugged coolant sensor turn key on reads same 417. plugged back in moved truck closer to garage warn went out coolant read 115 deg. unplugged again just to check warn comes on and 417 again. plugged back in temp jumpped back and forth about 20 seconds fan off and on then back to normal 115 deg.
my question is, is that what a no sensor should read unplugged (417) or should it read zero unplugged key on cm 871 deman.