Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
Allow appliance to cool
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if necessary (tap middle button on the fascia).
Apply a demand, does the boiler fire or does an overheat related fault code re appear without the boiler firing?
CLICK HERE IF A DIFFERENT ERROR CODE IS NOW SHOWN