LF 
Multiple causes 
Could be 
Ignition electrode 173528 
Detection electrode 173529 
Detection lead 173512 
Spark generator 173538 
Condensate trap 174244 
Flueing issues 
Gas supply problems
Wiring harness problems 
Or maybe even the pcb 
The list of possible causes for LF are numerous
& each one requires a different strategy to test it 
The boiler manual goes into great detail as to how try to test each component
Provided you have a multimeter a flue gas analyser & a manometer
                
 Q
                
 September 2015