Sender is just a thermistor(temperature variable resistor) inside it, resistance decreases as sender gets hotter. Could be it is the wrong value range to match your gauge, or you have a short to ground. If it reads cold with key on and engine not running and cold, but goes to hot too quickly when engine is started I would expect wrong range or faulty sender. If it goes to pegged hot as soon as power is turned on or engine is started, I would look for a dead short in sender to guage wire or guage wired wrong.
You could check it with a temporary test install of a mechanical gauge from the local Autozone, Napa, etc, or check the block and head castings temp with an infrared gun(laser thermometer)