So you're assuming there is a problem that is causing low MPG?
this morn my mrs's car read -3c on her car focus 1.6 diesel 2012 on mine it read 0c and when the car is warmed up the temp needle pointing to normal (12 o clock position),if it throttle it hard it needle goes back to (10 o clock position) then it goes back to normal.it defo has to be electrics now,anyone with idea of what it could be..
It could be that your computer is wildly innaccurate, and you actually haven't lost much MPG (only that which is normal in winter). I wouldn't bother doing anything until you measure again with a manual fill up. I normally get low 50's in summer, but recently i've been getting very high 40's. However the trip used to be 0.1-0.2mpg out, but now it's showing 44mpg. Others have reported this problem as well.
Don't assume that your car has a major problem just because an innaccurate dashboard display says you do!