Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect mains voltage plug from pump then remove red clip to expose connections
If boiler serial number is Pre ACQ pump may have been replaced by a standard USP3. In this case unscrew connector to expose connections

Measurement:

Check for continuity (zero resistance) of the brown and blue wires
running from the disconnected plug to X1C on the PCB.
Inspect harness and connectors
for good overall condition.