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Heater only works using recirculation

I’ve noticed this winter that my Mazda 3’s heater is barely working using fresh air. It definitely gives out warmer air than the outside temperature, but not by a whole lot. If I turn on air recirculation, then the air becomes appropriately warm. What would cause the heater to work one way and not the other? Thanks for any input!

I’m guessing that the diverter valve in the heating system ductwork is jammed in postion, due either to something blocking it in the duct itself, or the actuator that controls it has given up the ghost.

Check the coolant level after the engine has cooled. Low coolant will affect heater performance.

A partially stuck-open thermostat can cause this too. Does your car have a coolant temperature gauge on the dashboard? If so, is it registering as high a temperature it used to, and is it getting to that temp as quickly after starting from a cold start in the AM?