I am also running a low temp, AGAIN, in my 2005 Silverado 5.3. I purchased it from original owner, who i thought told me everything, along with a stack of reciepts! After 6 weeks of below 0 temps and NO HEAT! I started replacing thermostats like they were underwear! In the end I was told by the 4th mechanic ...somewhere along the line the ? (I think I was told water pump? But dont remember, now 3 yrs later) some OTHER part that had been changed prior, required the thermostat to be changed to one that wasnt stock with the vehicle. After believing the first 2 thermostats were defective, and the one guy guessing what to put in, I found someone who took the time to look up the ?? (I wanna say water pump but not sure) and HE FINALLY put the ? for lack of a better word... CUSTOM thermostat (because it matched with the replacemennt part I cant remember.. that was preventing the stock thermostat from working. AS SOON AS HE PUT THE "WRONG thermostat in, I HAD HEAT!! FOR THE 1ST TIME IN 2 MONTHS OF 0 TEMPS!! Another 2 months later moved to Florida and never thought of it again, till 3 years later yesterday. The temp is low at 155° for 2 days now. Its March & only 65-80. So now.. Im not sure whats happening. Again.