It is blowing air that is semi cool but not cooling the house. I changed the filter 2 weeks ago but the thermostat said it was time to change it again. I changed it and the one being replaced was filthy already. It was a 3 month filter. Whay could be the problem and how do I check for the problem?

Thanks

You need some tools that you probably do not have and you also need to know how to use those tools.

Your outside condensing unit could be dirty and that you could back flush.

You could take the temperature of the air going into the unit inside and the air temperature of the supplys to see if you get the mythical 18 degrees TD (total differential)

Make sure the drain line from the inside coil is clear so all the condensate runs out. A blockage can cause high humidity and the feeling that the a/c unit is not performing correctly.

Othe than these items you will need to contact a local pro for a on site inspection.