how can i fix my sabre 28 boiler?

My sabre 28 boiler won't fully fire up. when I turn on the hot tap or heating it fires up for about 15 seconds and then locks out with the red light on. I reset and the same thing happens. the pressure is fine so I know that's not the problem. the boiler is 3 years old and I have never had any problems with it before. any ideas of what might be wrong and what I can do to fix it?

duncan
December 2010

refill boiler useing filling loop under boiler (braided pipe) to 1.5 bar on gauge

hi-spec plumbing
February 2007