I have a 03 cummins and it flashed p0117 which is my coolant temp sensor low voltage so i replaced it. I drove 120miles and checkguages came on. What i notices is when the truck is cold its fine, when its warmed up temp guage goes nutz it runs normal at195 degrees and it would shoot up to 245degrees and back to 195 and aometimes it will stay at 245 for awhile but its not actually overheating help me fix this guage please.

 

First check to see if you have air trapped in the engine. Remove the allen plug that is in the top of the head close to the thermostat. If no coolant comes out, fill the radiator with coolant until coolant comes out of the hole. Put the plug back in and finish filling the radiator.

If you have no air trapped, check the water pump. I had much the same problem with my van. I went through a new temp sending unit, new thermostat, radiator hoses and last checked the water pump. I had changed the water pump not long ago and did not think it could be bad, but the impeller was loose on the shaft.