Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove Gas Rail from boiler for inspection

Measurement:

Removing the Gas Rail:
First unplug the leads from both electrodes.
Unscrew the gas connection and four screws (two each side) and remove the Gas Rail from the boiler.
Thoroughly inspect the injectors along the Gas Rail for blockages or damage.
Note:
When refitting Gas Rail check gas connection for tightness.