Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to start.
After approximately 2 seconds measure voltage across the red and blue leads on the gas valve.
Expected voltage approx. 20vdc at first, then reduces to approx. 15vdc.
Voltage will only be present for approx. 5 seconds during the ignition phase the appliance will try to ignite 5 times before going to lockout.
Note: Lockout may occur after 5 attempts to ignite. Reset boiler if required.