2005 berlingo van engine diagnostic code p0170 and ecu lock out for reading?

I have a 2005 plated 1.9d van approx 55,000 on clock,i have had a diagnostic code taken from this van which is p0170(fuel trim)any idea's how to fix/what to replace?
I have cleaned out egr valve.
The light comes on intermintally and the van runs fine with no miss fire etc,i had cam belt/waterpump changed and that is when light started to come on,could it be down to timing?Although garage have told me that it was pinned up etc
I went to local citroen dealership for there so called professional service to be told that both their machines were unable to access a code as ecu has locked them out,something about possible wiring fault to ecu ,possible bad earth /wiring?Further diagnosis required but unable to quote a estimate of cost!? They also charged me for the privillage of this!,i told them that i have a previous fault code reading for van but are not interested
Any help or idea's