Out of the blue, my wife's car stopped pumping heat out of the heater. First thought was the thermostat. Changed it and bleed the system, but still only a bit of heat coming out. Car's gauge shows the temperture as normal (just under the half way mark). Her car is the higher end version with the electronic controls. Is there some common issue that can cause this? Fan works / blows fine, just next to no heat comes out.