Did you have this problem prior to changing the MFS
Why did you change the MFS
Did you use a new MFS or a used MFS
The fusses associated with the MFS are #3 in the Battery Junction Box and #13,23,26,27 and 28 in the Central Junction Box
If the problem occurred after you changed the MFS then its possible that the switch is faulty or when you changed it you disturbed other wiring.
Attached are two prints on testing the MFS
As stated by Tonydiv use your meter to test for Amps
See what the amp reading is and then remove the above fuses and re-test
If you need any other prints let me know