Symptom
Completely dead, no heat fan or display

Check: There is power to the peak circuit and the off peak supply circuit breaker is on or fuse for that circuit intact.

Check
Both the peak and off peak switches are in the On position. if not then switch on.

Check
The peak mains supply is present at the Quantum  mains input terminal block, terminals 3 and 4 (counting from the left) 3 being L and 4 being N (this is fed from the mains filter board, located behind the CC)

Check
Connection between the UI and CC is in place at both ends and undamaged. if it isn't terminate or replace damaged part.

Check
That 12VDC is present on the lead from the CC to the UI, whereas red is +ve and black is –ve.

Swap over UI and see if fault clears, if so the fault is with the UI, If not put the original part back.

Swap over CC and see if fault clears, if so the fault is with the CC, if not put the original part back.