scenic 1.6 starting problem

hi just had my dephaser unit and cam sensor replaced on my 05 scenic 1.6 petrol same problem has returned two weeks later i am now informed by same garage ( not main dealer ) that my crankshaft and oxygen sensors are now showing as fault on dianostics is this usual . problem was car would start tick over for 15 seconds then stop would run fine when engine had warmed up , then when engine cold fault would repeat..