With the OEM radiator, my truck could get very low if I wasn't watching it (at least a gallon or more low) and act the same way when it started to go. Especially when you add in the rear heater loop, which adds a hidden volume of either air or coolant. Do you have green dried coolant on the upper or lower side of the radiator? Eventually all plastic tanked radiators fail where the aluminum clamps to them. I was surprised at how much I could lose. While I was putting off changing my radiator last time, I think I was adding somewhere between a half to a full gallon each month.RogueFinder wrote: ↑Sat Jun 13, 2020 7:22 pm2005 "finder, 154000 miles. Started overheating on short drive during a hot day. Threw P1217 code also. When I ran the heater on max in an attempt to cool it so I could make it back home, little heat produced in cabin. Got it home, and decided to take a look at coolant levels - very low, less than a gallon after I drained it. No wonder it overheated. In the process of flushing, ran the engine at idle with the cabin heat on again, still very little heat coming from the vents; coolant temp after running for 10-15 minutes was 203-deg F. Running it with water and Blue Devil flush right now. The truck has history of temperature spiking on hot days and sitting at idle, like in stalled traffic or long drive-thru waits. But get it under control and in the normal range after normal driving resumed. Does this indicate a thermostat problem? Something else? I haven't seen any indications of leaking coolant either, how did I get so low? was changed last year. Also what triggers the smaller fan on the radiator to come on? This fan never came on when I idled it to flush.