Navigate to the "Flame Monitoring" section (Section E, Fault Code F102). Step 2: The manual tells you to measure the flame current. Minimum acceptable for the QRA2 photocell: 70 µA. Step 3: Clean the lens. If current stays < 50 µA, replace the sensor (part listed on page 6-33).
For more information on the Weishaupt G7 1-d, consult the following resources: Weishaupt G7 1-d Service Manual