FORD F150 V8 WITH VOGAS

[cranny]

First time on here, so here goes.
i have this 2003 Ford f150, 4.6 v8, i bought it with the vogas system already fitted and have had no problems till about 1 month ago.
The check light keeps lighting up, codes for bank1 and bank2 rear lambda sensors weak. I have normal switching front sensors, .1-.8mv but no read out on the rear sensors just 000, i woul have thought it would read like a normal petrol reading of .450mv if all was ok , seems funny why 2 should go at the same time, dont like the idea of £200 for 2 sensors if it could be something else.
Also i cant get the switchover to gas to work now, it keeps trying but car runs really rough and i have to go back on petrol, does any one if these will have any filters and where i may find them.
thanks,
cranny.