Today I learned my 2002 Duramax has two-yes-two cooling thermostats! They reside beside each other and use "O" rings to seal the housing to the block. Mine began leaking the day I received my truck back from the fuel supply line issue. The service writer, who is very courtious and helpful, said that strangly enough, everything, both thermostats, labor, and the sealing rings will be replaced under the engine warranty. I must wait until Friday though, he said there are no Duramax thermostats or sealing rings in the state of Indiana, they must be ordered from elsewhere. Now my question is: Why does the Duramax engine have two thermostats at all; and sitting side-by-side? I speculate this if for coolant flow volume, and if so, could one faulty thermostat reducing flow be the reason for some overheating problems posted here at DieselPlace? :banghead:

More flow
redundant if one fails

There are 2 different temp ratings on the stats.

I supect that as desired engine temp is reached, coolant temp is better controlled by 2 stats. At the low end of the desired temp, coolant flow is minimal and at the high end, high coolant volume through 2 stats is far superior.

I guess a bad stat could be a reason for overheating issues, but with one working, pretty hard to diagnose.

Just returned from dealer. The explanation is: each thermostat has a different opening temperature. This is part of the quick warm-up feature briefly mentioned in the owners manual. The lowest opening stat in for quicker warm-up and unloaded driving. The second higher temp stat is for heavy loads on the engine, permiting higher coolant flow rates that under unloaded conditions will be excessive flow. I can't vouch for this explanation, just repeating it as best by memory can.