Doesn't sound good to me. I would definitely replace it (will have to eventually anyway). There is a separate valve control that sends the water to the heater core when heater is switched on but if it is not getting hot then the water pump is probably not circulating properly. Thermostats rarely go bad unless they are very old but you can replace it at the same time to kill two birds just in case. Fortunately these waterpumps are easier to replace than most but are costly due to being electric. If your fan is cranking on to high speed the computer is sensing a need to cool quickly which typically means engine is too hot and poor circulation. 40K Miles is pretty low for a bad pump though. You are under warranty so just take it in!
Last edited by sptt144; 11-18-2012 at 12:48 AM.