Set Appliance State:

Mains Power Off
Ensure boiler has cooled
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Press and hold the reset button for 5 seconds then release.
Apply a CH demand.
Does the burner relight or does the appliance return to overheat lockout fault code?
Note:
If a fault code other than overheat lockout is displayed CLICK HERE