How can i fix the pump?

I have a Bosh WFK 2401. It washes the clothes perfectly fine but when it comes to the spin function it doesn't get rid of the water and the code 03 appears on the panel. I drained the pump several times and checked if the pump is rotating freely and that seems to be working fine but the pump is still not functioning properly and so it only works every now and then. How can I fix this problem