How can I fix my Vaillant Ecotec 831 boiler?

How can I fix my parents Vaillant Ecotec 831 boiler? It is nearly 2 years old and has failed numerous times since it was fitted. It has now "conked out" on them every day since Saturday. The guy who fitted it has been out numerous times and fixes it for a few days and then it fails again. My parents are elderly and have my handicapped brother to look after so naturally I want to help them sort this out before winter takes hold. When the boiler is switched on it makes a groaning sound like it's really struggling. There is no heating and no hot water - can anyone help? Many thanks.