Ferroli Domina trouble

Hi,
I'd like some help with a Ferroli Domina N, please. The boiler ignites, for CH, stays lit for about 1 - 10 seconds, and then goes out. For HW the boiler was doing the same, but now it's stopped lighting at all. It's a fairly new installation (18 months). I've checked the thermistor resistances - they're ok, according to the installation guide. The air switch is OK. The fan runs. I've checked the gas inlet pressure and the max and min running pressures and they're ok.

I'm running out of ideas - can ayone help? Do I need to replace the pcb?

Thanks,
Wander.

Have you checked the voltage at the gas valve throughout this process? Have you tried by-passing overheat stat?

the pump runs ok ?

Thanks for your suggestions. I think I've found the problem - the CH thermistor is intermittently failing. Took a while to find because when I poked with the DVM probes it seemed ok...