Temp gauge pegs out then back to normal
 

The temp gauge on my '05 Cummins pegs out then returns to normal . This has happened 4 or 5 times and each time when the temp goes up I hear a warning beep but no warning lights come on the dashboard . Have noticed that the truck temp fluctuates within about 20 or so degres both ways during normal driving . Doesnt seem to be anything consistent when the temp pegs out . Any help with this would be appreciated !

Replace the thermostat.

Agree. Thermostat would be my guess too.

That or the temp probe, but I think I'd also start wit the thermostat.

If the truck is 5 years old, it is due for a coolant change anyway.

Thermostat is first guess, the sensor is getting dry or a loose connection are other guesses. I'm not sure where the temp sensor is, but if the sensor is high on the block/head and your coolant is low or there is air in the system, the sensor gets uncovered and will read high until the coolant covers it again. Air in the system can come from the water pump seals or head gasket leaks.