Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect leads from fan assembly (note connections for reinstatement)

Measurement:

Measure for resistance across fan terminals.
Expected resistance readings should conform to the appropriate table below:
24 Si 2
TerminalResistance in Ohms
Black - Red(approx.) 48 Ohms
Black - White(approx.) 128 Ohms

28 Si 2
TerminalResistance in Ohms
Black - Red(approx.) 27 Ohms
Black - White(approx.) 128 Ohms
                         (for RD628 use 28kW values)