Engine Check Light on constantly runs ok

Hi all, yesterday my wife informed me that the Check engine light had came on on my 300tdi auto with EDC, The light has stayed on since regardless of what the car is doing. switching it off and re-starting does not make it go out. Acording to her the car still is still running and driving OK. Just wondering if anybody has had a similar problem? I cant get any more details as i'm away working so have not had a look over it. I do know that the ABS light has permernatly been iluminated since the day before I left for work. but dont see these two being related. the last things done to the old girl was to change the exughst and remove the cat. she has just been serviced and i re-oiled the K and N filter, could this oil have contaminated the MAF? think the ABS light is proberbly a sensor coz I just did the front wheel bearings. The car has not been used much since all this work so guess both problems are related.

any info or lead you guys can give would be helpfull, as i'm not home for anougher week and better to pinpoint where to look or at least narrow down the search,
Cheers