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by bagicide stayed 10 months too long on January 18th, 2009
It is possible for a replacement thermostat to be bad. I think I'd take it back out and test it. We are talking about a really simple system here. The engine builds up heat as a byproduct of combustion and coolant carries it away. When the engine gets warm enough to run efficiently, the thermostat opens and coolant begins to circulate through the heater core as well. When you "turn on the heat" the blower motor begins to circulate cabin air through the heater core and blow it into the cabin. So basically, there aren't a whole lot of options. You know your fan is working, because it is circulating air. As long as there is coolant in the system and the thermostat is opening, there isn't much else, unless there is enough crud in the system that it blocked the heater core. I guess that could be a possibility in a ten year old car, that there is just enough corrosion in the heater core to block it, but before I'd replace it, I'd test that replacement thermostat and make sure it wasn't defective from the factory. I've seen that happen.
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