Persistent fD Fault despite pressure reading correctly

[green5]

My heating system is a 'heat only' boiler with a pressurised cylinder, Grundfos pump and separate upstairs and downstairs zones.

I've drained down the system and added a new leg to my central heating for a new radiator in an upstairs bedroom.

When I've tried to refill the system the system is coming up to pressure on the gauge but the boiler is reading an FD Fault.

I've bled all of the radiators and I've ran the run off valve with the filling valve open but the problem persists. The pump seems to be working.

Any ideas of what to try next?

 

[CountryFan]

What make and model of boiler? Trapped air?

 

[green5]

It's an ideal logic boiler.

I'm presuming it's trapped air but having bled all of the radiators I'm not sure how to release it.