My 2004 denali xl has a strange temperature gauge issue. First of all, it runs great, has 139k, I've owned it since 2009. I changed the water pump, thermostat and all coolant hoses at 103k when the water pump started leaking. What happens is the coolant gauge will show normal temp, just left of straight up, once the truck is warmed up. Upon restarting, once warmed up, the gauge indicates anywhere from 240 to 260 degrees. Everything under the hood seems normal..coolant is topped off, no leaks, no lights on dash. If i disconnect the battery once stopped, the gauge resets and doesn't present the problem until multiple start/shutoff cycles. It will run at normal temp the first time after resetting the battery, then it happens again.. I'm thinking the coolant temperature sensor, i have no scanner, so i don't know about codes..any ideas?