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The cooling system has very little work to do in the winter because the air passing through the radiator is so vastly cooler than the engine. Consequently, less than half the normal operating capacity of coolant is sufficient to keep the engine cool. However it is NOT sufficient to go through the heater hoses in many cases. I've even seen "no heat" as a prelude to overheating in the case of a leak. Plus, let's face it, this is a fairly simple thing to go check. So see if the car's full of water already! Even easier than changing the thermostat. "Low coolant = low heat" refers to the heater, not the engine temp. If the guage reads low, that really points to the thermostat and not much else save the guage circuit and components themselves.