Set Appliance State:

Mains Power Off
On/Off switch ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the appliance reset and begin its ignition sequence or does the F13 fault code reappear?
Boiler resets - retest appliance
Boiler will not reset or resets and returns to fault code
Note:
F13 can often be intermittent. If you wish to investigate an intermittent
F13 fault code select "Boiler will not reset or resets and returns to fault code".