Bought the car some months ago and it was getting about 45mpg, now this has gone down to around 30mpg. The exhaust fumes have become a lot heavier and the engine fault light comes on some times, otherwise the car works fine. A friend of cars says it needs a computer to test it. Does this seem right, who can do this and is it costly? I'm unemployed at present and can't afford a lot of petrol!