Worcester Greenstar 18Ri Volatile Lockout...which sensor to replace?

I have a Worcester Greenstar 18Ri boiler which runs for about 60 seconds before stopping with a fault code of fast flash. This apparently means volatile lockout and is down to either the temperature sensor or its wiring or the code plug. Given that the boiler was working fine before, the code plug is unlikely to be the cause of the problem and I have checked the wiring which leads me to believe that the problem lies with the temperature sensor itself which I will now replace. The problem arises in figuring which sensor Worcester mean as there are 3. There is the Control Primary Temperature Sensor, the Flue Overheat Thermostat and the Overheat Thermostat...any help would be appreciated.

Andrew Frigot
October 2011