Whenever the air filter is full of dust and dirt, it concentrates particles from the interior air supply, but less air passes in. Due to this, the air conditioning system will have less air to condition. Your air conditioner may read 74 degrees, but the temperature will never fall below 80 degrees. If your air conditioner alternates between blowing cool and lukewarm air, your temperature sensor may get set to “on.” Set the heating system to “auto” mode to see if it makes a difference.