Subaru Repair: Outback Engine Check Light, catalytic convertors, timing belts

Question
I have a 1999 2.5 Outback with 105K miles. I have just returned from a long trip. After 200 miles the Engine Check Light came on, whilst on the drive home it came on much sooner. When away the light did not come on, though the car was used sparingly and for short-ish distances. The common factor being just over 70 mph and had to keep the at this speed all the way home. Though it did take 200 miles at 80-90 before it did initially appear. It was just serviced prior to the trip. The parts used were generic apart from timing belt, which is Subaru. Could anything have happened during the service to cause this. I do intend to take it to a dealer for the code and corrected, however my brother in law did the service and if it's something he could fix would rather he did it, plus I need to tread carefully as to not offend him as he is pretty clued up, he's also a lot bigger than me. Any ballpark suggestions would be great.
Stuart.

Answer
Stuart,

There are just way to many possibilities to list.Yes aftermarket parts can cause problems and yes its possible that its not related at all to any of the work that was done.MOST LIKELY there is a condition that has been created with the service that the ECM does not like.Sometimes when you replace worn tune parts and stretched timing belts you will make the engine perform better and in the course of doing this you can expose conditions such as poor performing catalytic convertors.I wouldn't assume something was done wrong only that the car has many systems that affect each other and while improving one you may negatively affect another and the answer is to address problems as they arise.

Justin
All Wheel Drive Auto