2006 Vauxhall Vectra starting.white smoke.p0380

Car doesnt starting from 2-3 time every morning or then stays cool down more then 3-4 houres. White smoke comes from the back only when its hard to start. Summer time its was better but now getting worse with cold weather. Been connected to pc and shows p0380 error. Glow plugs been checked and all working.. cables all fine .. its just telling that 1V coming instead of 12V as it should be. Cant find a fault.. please help!