Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB (use Insulation Mat as required)
Disconnect flame sense lead from CN900 on the PCB
(leave in a safe position)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance (press reset button for 2 seconds and release).
Does the boiler now reset?
Note:
1. If after boiler reset the fault code changes from the original to a different fault code, then CLICK HERE.
2. Reset is confirmed by the display returning to show ON displayed when NO demands are present.