Hello. I have just fitted a brand new Ariston fast r onm11-14. Tries to light up then goes out. Any ideas please.

What type of gas are you using?... depending on what type of gas, the boiler must be fitted with the right injectors for Butane(GPL) or Natural Gas(see specification sticker on your boiler and in the link attached here under). ¿How much of water pressure?... Check your installation Manual to make sure you are inside the limits of use. If flame is going off after ignition, you might have a problem with the flame sensor or not enough gas/water pressure.

If this boiler is totally new, you still have factory guarantee,  contact the manufacturer,s SAT and make use of free assistance. Obviously, you might have to pay for transportation to your own location.

https://www.interempresas.net/FeriaVirtual/Catalogos_y_documentos/171382/143_204_manual_instalacion_y_uso_Fast_Premium.pdf 

¡Gracias! Thank for the info. got engineer to come from manufacturer day after it was fitted. it turned out to be a temperature sensor that was open circuit.  They replaced this and it all worked fine. no cost  to me . thanks for the help