There could be a number of causes but the most common is low system water pressure thep ressure quage on the front of the boiler should read around 1,2 to 1,5 when the boiler is cold
if the presure is much lower then you need to top up the system using the fillling point under the boiler chrome hose with tap open slowley and fill to the above pressure reset controls and re try there may be a seperate reset button to press on or around the control panel
steve@ www.thermagasplc.com, March 2007