Any other symptoms like low water pressure? What model is it?
Kerdevan
March 2006
check knobs on front boiler are not broken in to pcb board they may feel ok but are not connecting with pcb .may also need service kit as spindle between diverter and synchron may be sticking.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.