potterton suprima 50 pcb problem?

after several years of problems,2 pcb boards,pcb repaired twice since [dry joint problems] new upgraded pcb fitted ,to end the problem,it now only works the hot water system,but not the heating,any ideas how i can check out which part of the system is not operating correctly?which colour wires have what function and what current? how can i bridge out thermostat? what can i do to check out diverter valve?