"Suddenly Missing Fuel Economy" --Bad Thermostats?

Something just dawned on me while I was staring at the Fuel Economy page with a bunch of threads posted by people who think something is wrong with their car due to a steady decrease in fuel economy. Granted, it's coming up on winter and we expect a significant drop-off as temps decrease. (I've logged tanks in the low 40mpg range due to weather.) But it dawned on me that an ancient bit of technology could be the culprit: the thermostat.

This thread has been discussing the relative merits of an idiot light vs. a temperature gauge to indicate over-temp conditions. But there's no reason to expect that it would alert anybody of an undertemp condition. I have personally seen what happens to a "normal" car when a thermostat fails open in cool weather (dumping coolant to the radiator) and it isn't pretty: poor running, lousy cabin heat, and very bad gas mileage. In a Prius the effects would be exacerbated since the hybrid drive system wouldn't allow the ICE to shut down since the engine can't get up to temp. It's possible that this condition would produce a CEL and a code, but I don't know. Does anybody know? And has anybody with a mysterious and inexplicable loss of fuel economy (preferably someone who has already been through a season or two and has seen normal fluctuations before) tried tracking coolant temp with a ScanGauge or other device?

 

Aww, shucks. I hoped I was onto something. Thanks for the succinct response!