Ok, it's winter in NH, but even after half an hour of driving at night, the temp gauge is barely above the cold marker and the heat isn't so hot (but it works, more or less). Hoses were replaced this past fall, as was coolant, and the level was double checked since noticing this problem.

I know it's an aluminum engine and all, but is there another thing worth looking at? Does this suggest a bad thermostat? How much of a PITA is that to replace? Thanks,

 

Most likely the thermostat, these where prone to sticking in either open or closed postitions, and yours seem to be open. Very easy to change, if you can change hoses you can change the T-stat.

 

It definitely sounds like a thermostat. Be thankful it stuck open rather than closed. Too cool is much better than too hot. My wife's thermostat has been stuck open since last spring. It wasn't a problem over the summer, but now that it is cold again her check engine light is on. I have the part, but apparently changing a Mazda3 thermostat is a PITA and I keep putting it off.

 

Yup.. thermostat...

 

Thermostat is the most likely cause, but extremely low coolant level will appear to do the same thing, not enough to fill the heater core and not enough to reach the coolant probe to indicate actual temperature.

Might be worthwhile running through the purging/bleeding procedures just to be sure.