All oil boilers check for a stray/false flame before allowing the boiler to fire up, so if you have the cell out and you have a light shining on it before you press the start button then it shouldn't fire up, can you check this out.cell works exaclty as it should when out but locks out everytime when in position grant vortex 16/21 riello burner
To fire up with the cell removed, it has to be covered during the purge period which on my 17 year old Firebird (Riello G5X burner) is 12 secs, the ignition transformer is then energised and the solenoid opens the oil valve and a flame must be detected within 5 secs, so if you shine the light on it after 12 secs it may light up and allow the burner to continue firing, For safety reasons you obviously don't keep it running in this fashion, as soon as you remove the light or blind the cell then the burner should trip with the combustion failure light illuminated.