Red light?

I have a sabre he boiler..the red lights keeps comming on I have done all the usual things switching boiler off from mains turn mode to off position then back to setting nothing happens... Red light can be on for hours result no hot water/heating then it will turn to green for a short time .... This problem is intermittent .... Would you know what causes this to happen cause I have had 3 engineers out and because by the time they have arrived it's gone to the green light so they say no fault!!!! And not able to pin point the problem. Can you help please ....