l just bought a second-hand Citroen C3, 13000km...
The thing is that the ''Check Engine'' signal lights up everytime l start the car. The car seems to function _perfectly_ fine but l am still concerned because that light SHOULD not be on. Does anybody know the list of reasons that can cause it to be on?

And, if somebody knows where l can find a user manual, I'd really appreciate it.(Both english and spanish are fine)

The check engine light is telling u that the engine computer has picked up a defect in one of the circuits that monitors engine function. Often it is as simple as an oxygen sensor that is not quite up to standards, or could be more important repair. Sometimes as well can be just a wire unplugged. Since u bought it second hand could be anything since u do not know the whole history of the car firsthand. A simple diagnostic test at a garage wil turn up ''codes'' in the computer that caused the light to come on. It is this code, or codes that will show what has appeared to malfunction & cause the light to come on. Normally if the car appears to drive fine otherwise then oxygen sensor or other simple part is to blame & the trouble is cheap to fix. l would get it checked soon though as running too long can cause other parts to fail & also ur fuel mileage can suffer as well.

try buying a HANES manual from ebay or go to the dealers & let them have a look at it so that they can sort it 4 u