Here's a tip on how to verify cartridge plugging.

  1. Remove all filters
  2. Open the inlet valve and see how much water pours out of the 1st filter head.
  3. Fit the paper filter, open the valve and see how much pours out the 2nd filter head.
  4. Remove the paper filter and fit the poly...
  5. Remove the ploy and fit the carbon...

This process clearly shows the amount of plugging for each cartridge.