Procombi 85 HE hot water ok, but central heating not working?

The boiler is working when I run hot water, it fires up when the central heating is turned on, the pump get's red hot, the flow pipe is luke warm, but it turns off after 10 seconds, the fan is working. I have drained the boiler, but nothing has changed. I spoke to the help line although he could only suggest replacing the PCB, but as the water works, I think it could be something else?