This is for F7 code. If this procedure doesn't work inspect the PCB for dry joints, but this will probably work:
Switch off boiler. Pull off the three connectors to the spark plugs and sensor, making sure they can be identified correctly when reconnecting. Unscrew the two bolts to remove (carefully) the spark assembly and viewing window.
Clean the sensor and electrodes with sandpaper. Remove any dust. Replace panel and connectors.
Steven Black, March 2017
