it sucks,im a tech at the dealer and hear this every day,but as soon as a couple of us figure it out in the shop i'll post for you guys
My new company truck is an 09 Chevy with the same block heater cord. The twist to this is, that it is tied into the computer and the computer fouls up when plugged in. My dealer advises me not to plug in my truck until GM get a fix to the problem. Not such a great think with the -40C temps we've been seeing. I asked if I could replace the cord with a standard code and she tells me the computer will come up with a fault code and give me problems. And to think that GM is asking the government for a bail out.
So what is the reason for the thermostat?
I just had this entire thing happen to me! The cord end got damaged and I replaced it with a regular end. I plug in every night and drive to work where it's plugged in for 12hrs. All is well until the ambient temp comes above -18. Then the ECM sees that the the water temp is above the ambient temp and it's been sitting so they should be the same, so it creates the temp sensor fault code. That turns on the cooling fans, shuts off the A/C and a message on the dash saying so. That is all fine and I can live with that, going Back and forth to work , and only starting it cold, HOWEVER, if you try to restart it after it has warmed up, it will flood because it has gone into de-rate or "limp mode" because it now believes there is a engine temp signal that is incorrect and it's doing everything it can to protect the engine from overheating. Hence the fans on and the extra fuel, which works on a cold engine to start but not a hot one. That is how I understand the issue. Some ppl in Winnipeg are tieing the cord thermostat to a rad hose to make it work correctly down toward -50c. With the thermostat cord end in the grill I never had any trouble in Alberta down to -40c. So long story short, you need the thermostatic cord end or there will be codes and assorted problems.. who knew.
Hope this helps somebody!