Zafira Sri150 1.9 diesel ?

Hi My 2008 zafira just had a new DPF fitted 1st day was fine but coming down the motorway engine management light has come back on. It's a 6speed auto in 1st and 2nd seems very sluggish but when goes into 3rd seems normal, can anyone suggest why engine management come straight on, before I take back to garage to be plugged in. Many Thanks