pentec

My dad is having an issue with his 2009 Sierra 4.3l. This message pops up on his dash every so often. Sometimes it goes away in a day or, sometimes it is on for weeks. I replaced to coolant temp sensor today and the message turned off almost immediately upon starting the truck. The weird thing is that I hooked up a code scanner that shows operating info as well and the temps were reading different. The truck warmed up to the 210 mark on the dash gauge but on the scan tool it never went above 180. The message on the dash has not come back yet but I only changed it this morning. Thanks for any help.