its an easy check to pull the fan and see that its spinning freely. Like Gp said they are not expensive to replace. The best thing I did to improve the output was to tape all the joints together I could reach when I last swapped out the head unit and the highs I mounted in the AC vents. I also closed up the gap around the heater core and AC where it comes out of the suitcase. The factory used foam in this area which was OK but leaked and was mostly missing on any car I came across. You could feel the air pushing out when the fans were on. Friend of mine turned me on to using electricians putty. They use it when running plastic conduit thru walls to seal up the hole and keep the rodents out. It doesn't get hard, dry out, or promote mould. I sealed up both of the openings tight and the output into the car is noticeably better