I have a 2007 Focus 1.8 tdci. The car started on Saturday morning then cut out. An Engine Systems Fault message came up on the dash board. I continued to try and start the car without any success. Britannia rescue said it could be the immobilser but they weren't sure (the guy had been working 12 straight hours dealing with cars caught in the snow on Friday and I think he was on his last legs) so the car was taken to my local garage for analysis. A number of fault codes are coming up but the one that remains when reset is P0091. Anyone any ideas what this fault means?