This relates to GP36 and GP42 (all variants).
Error codes F21 to F26 relate to faults in either the relays on the control board (K1 to K6), the lamp group contactors (Confusingly, also referred to as K1 to K3), or the cabling between. There is a diagnostic plug to help narrow the fault further.
The relays on the control board are not designed to be replaced, it is a control-board swap-out in this case.
Dave