Worcester Greenstar Ri keeps cutting out?

My boiler keeps cutting out. The mains (blue light) is on and the boiler fires up (green light on), but after about 3 mins the green light goes out totally. There is no flashing light. The blue light stays on. Is this a frozen condensing pipe or something else? (Last year we had the frozen condensing pipe and the green light flashed). Please help - we are getting very cold.