Ferroli Modena 102 lock out?

Led 2 light on boiler, is burner shut down and is constant red,after resetting boiler with CH at max it tries to fire up, but after several seconds switch`s back off. The boiler is not sparking at any time, have replaced the safety thermostat, checked sensor and ignition electrodes for damage all okay and replaced the PCB, and all connections to PCB are good ,gas supply is there before the gas valve, but no gas is getting to the combustion chamber at any time.
Any advise to what the problem might be?