Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Carefully remove burner from boiler.

Measurement:

Inspect the detection electrode position.
Place a straight edge across the top of the heat exchanger above the detection electrode and ensure the electrode is 12.5 mm +/- 1mm below the edge.
NOTE.
1. Ensure that when removed care is taken not to damage the
ceramic face of the burner.
2. Before reassembly inspect all gaskets and replace if damaged.