Coolant temp sensor

I had to replace a gasket for my exhaust manifold and unplugged my coolant temperature sensor sensor and after I was finished I plugged it back in. I turned the ignition on and the coolant temp gauge was reading maxed out even tho the truck was cold. I unplugged the sensor and same reading on the gauge. Truck is throwing a p1258 overheating code and dash is saying limp mode overheating. I inspected the wires all the way to the y and they look good. I replaced the sensor and nothing changed.