How can we fix our Halstead High Ace Combi boiler?

We have been having problems with our combi boiler. We have flushed the system out and it worked for a few days, Now it will not stay alight the light flickers then goes off and will not light it to able it to fire up. We have fitted a new detectionelectrode but it still we not stay on and light up.
Please can anyone suggest what to do as we have no hot water or heating