Intermitant engine check light and vt ls1 diff to vs

Just got a few quick questions please!
The vs calais (v6) has had the engine check light come on and stays on untill you turn the car off then when you restart it its gone? done fault code check and all systems are normal also when the check light is on the car runs fine! anyone have any ideas?
Also im wanting to do some work to this car by droping a ls1 into it @ some point and as such am going to use the vt series 1 diff as they are supposed to bolt straight up ect but im concerned about the abs sensors as im not sure but did the series one diff have the sensors? cheers!

 

bugger no one know?