Why does my boiler go g-g-donk and then lock out?

This answer is from the guy who put the question in the first place. My boiler is now working perfectly.

I found 2 problems. First I suspected there might be air in the system, so I replaced the air separator. The old one was well and truly gunged up, so it might not have been working properly, but it didn't fix the symptom.

Second I bought some descaler from my local boiler repair place and descaled the secondary heat exchanger. Loads of black stuff came out of it. Bingo - problem fixed!

Tony
February 2009