Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Fit temperature probe to flue gas test point,
ensure it is fitted securely to prevent escape of flue gases
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow burner to ignite and establish.
Monitor the temperature of the flue gases until the temperature stabilises.
Is the temperature of the flue gases approximately 130°C or less?
Does fault code F04 occur during the test?