red light constantly on vokera combi boiler?

The red light on my vokera compact HE combi boiler is constantly on. As per the manual it could be due to system overheating or NTC short circuit. I tried to repressure the system by adding more water hence taking the bar pressure to 1.5 bar during that light went back to green & when i switched on the central heating it seemed as if boiler has fired on but it revereted back to red light mode with in couple of minutes. I need help as no hot water or central heating.