Engine light on dash

[Wilkyg]

morning all  my van went into limp mode the other day with a spanner on dash and says service required, but van has just been done took it to garage i use and stuck it on dionostic and no fault was found van ran ok for four days then this morning same again lucky was at garage put back on machine no fault any idea what could be causing problem only had van since July and payed 4k but only got 30days warranty could I still take it back and say not happy .