I got engine warning light after getting De Cat fitted last summer.  It didnt worry me cause I knew it was because of the de cat and the light can be mapped out.
However....
When I put the cat back on for the TUV / MOT test Im continuing to get the warning light on the dash.  I had the car scanned today at VW (For free  

) and the result was lambda fault.  
 
  €250 for a new lambda.
**off to scan the print out to see if there are any 2nd opinions...