Hi All, My '07 Accord 4cyl threw a p0128 code last week indicating "Coolant Temperature Below Thermostat Regulating Temperature." I hooked up my scan gauge and and on a 20f day, with about a 2 minute idle before leaving the house, the coolant temp was up to about 175f within about 5 minutes or two miles of driving and the car was throwing decent heat. Temps for the most part stayed between 170 and 180 for the rest of the commute with the exception of when I rolled down a 1/2 mile hill in top gear (5mt) and the temp dropped to about 165f. I believe the car has a 170f thermostat from the factory. Ran a test last night after letting the car cool for about 2 1/2 hours by starting it and letting it idle under no load with the heat off. Ambient temp was about 15f, water temp was about 75f. Within 10 minutes the water temp was up to 175 and by 15 minutes the water temp was up to 185. I then turned the heat on full. After about 5 minutes the water temp was back down to 147f. Does this sound like a thermostat is stuck open, or is this somewhat normal for a small engine in cold temps being asked to put out heat without much load? I noticed last winter that the temp gauge would start to drop when rolling down long hills, but it always kept throwing heat and never threw a code so I didn't think too much of it. Also, the main reason I am asking before just changing it out is because this is the first code I have even encountered that I cannot clear with my code reader. It says command sent, but codes remain. I don't want to change it out if this is more indicative of a bad sensor or something else. Any help/input appreciated.