Air with little flow

Hello everyone and thanks in advance
I have a Fiat Doblo 2004 in which the air comes out without force: Angry :, makes the noise of always
but so that it is noticed you have to put it in the 4 when I always had it on the 2
I think it has to have some filter or something obstructed, can that be that? , how do I get to the filter to capitalize? .
Thanks again

Acasegra I have a doubt with your problem, the ruletilla in the two no longer throws you or throws very little?

It does not leave air that you have to put it in the 4

Look if this is how you say it, the rule is checked when they spoil the speeds only the fourth is the direct.

Thank you very much Kikemec, but how could you check the rule?