Since I am not sure what is wrong I will try posting here. The temp gauge in my LB7 intermittently goes way past 210 coming close to the red mark at 260. This began about a month ago, happened three times going freeway speeds in the morning (air temp about 45, not hauling anything). Took it into the dealer who could find nothing, including checking the gauge cluster. I never got any warning lights or codes. I took a chance and had them replace both thermostats and went almost three weeks without a problem. This morning it did it again. As I pulled up to work I shut the truck down for about 5 seconds, restarted and the gauge read one mark below 210, which is about where it usually runs. Prior to these episodes, I had never seen it above 210, even when 115 here in Phoenix, pulling my Diamond T with the trailer (10.5k pounds). What do you all suggest?

What about the water pump???

Sending unit?. Lose connection?

Thanks to those who replied and tried to help. It turned out it was the gauge and they ended up replacing the whole cluster (I had an extended warranty that covered it). The gauge would read warm after sitting for two days and then finally wouldn't read any temp at all, after it had been driven. The sender was OK, the dealer (shop foreman) said they have had to occasionally replace clusters but never the sender.

Hey DrZ, where in the Valley did you get your service?

I bought and have had my truck serviced at Chapman's in Tempe. They seem competent. I have heard, through a friend who is a Duramax engineer at the GM Proving Grounds here in the east valley, that the techs at Thorobred in Chandler, are OK, too.