Impossible to tell how trivial or serious it is. For that, you'd need a proper diagnostic code reader and (just as importantly) someone competent in actually working out what the fault is from the codes generated.

It's entirely possible that the vendor has had a light on in the past, and has just cencelled it with a code reader prior to sale. However, such action would be virtually impossible to prove, and, in any event, a private sale is 'caveat emptor', so you'd have no or minimal rights against them even if that was the case.

Do be aware that, as of the 20th May, and engine management light that does not extiguish itself on startup will be an MOT failure.

Any number of things, I was very careful looking up ford focuses when I was buying my next car a year ago, (I wanted a ford focus) actively checking MOT histories and whatnot, the car I chosen was relatively free of failures on its MOT history and it came with a years MOT.

I test drove the car, nothing seemed out of the ordinary so I purchased the car. 4 miles later the EML came on (yes FOUR MILES LATER), I took it to a garage, they couldnt see anything wrong and erased the fault.

Cue several months and FIVE EML lights on later with said garage just telling me it was an error and thus deleting said code, I got fed up, I took it to another garage, they said that the cars VVT (variable valve timing) wasnt working, I'd need a new VVT valve which also meant a new cambelt and water pump since the whole system were interlinked together, £1000 to fix. I paid to fix it, and no EML since, and the car actually accelerates better now than before the fix. I can now go up Telegraph Hill on the A380 in Devon in fifth gear at 75mph.