2.0 tdci p0191 not clearing

hi folks.

i have a 2010 2.0tdci with the d4204t which has cranked over a lot before firing up since i got it.

a lot of bubbles in the fuel line leaving the fuel filter which I have recently replaced and seems to 

have helped but not completely eliminated the air in line. i no longer get occasional bubbles at idle

now it is only when i rev up the engine.

i pressurized the fuel tank slightly but didn't manage to find the point of air ingress yet.

Car drove with plenty of power despite the air in the system until recently when the engine malfunction message appeared.

the codes (found via TEST) are abs/esp related due to a fault in the PCM .

I just got my first use on forscan yesterday and the PCM has DTC for p0191 fuel pressure but it is not displaying a MIL for this.

 I can't clear this DTC however it stays logged on the PCM with ignition on engine off which I find odd.

While looking at other PIDs i see that although 3 PIDs for accelerator look ok the Throttle PID goes to WOT once the pedal 

is pressed past closed throttle (CT). I only see it change to PT on engine overrun briefly.

I am getting some lines soon to do an injector leak off test. however the rail pressure does not drop away that quickly after shutdown.

any thoughts on why the DTC stays logged after a reset.?