The following is just what i think and i have nothing to back it up with. the c5 has a adaptive drive cycle where the ecu learns your driving style and alters/adjusts things to suit your driving style and legal emmisions. so for the last few months your car has done mostly town driving at english temperatures using english fuel. then you put a caravan on the back and head of down the motorway.to hotter climates which is totally alien to what the ecu has learnt. we o.ften get this on the taxis where they spend months round the town then go off down the country and the management light comes on, Now remember we have experience also free labour and a abundence of spare parts and we can never find out what puts the light on. what we have done in the past is before a car heads of on a long distance ( i regulary take one to spijkenesse in holland) we do a bsi reset and the car then learns the driving style on the motorway and dosnt put the light on but what it does do is when the car is back to work round the town it then puts the engine light on and again theres no faults to find. this happens on whatever c5 i take. Like i say i have nothing to back this up with at all just a opinion