Some information that might help somebody, somewhere who knows...

Ok,My car is MY00.
CEL came on and stayed on for a week. Also between 3000 and 5000 revs juddering and engine being lumpy.

Took it to a Subaru garage who changed the cam sensor, cost 100 quid, thankfully under warrenty!

45km later CEL came on.

Back to Subaru who changed the O2 sensor. CEL gone but still lumpy.
100km later CEL came on.
Back in to the garage, this time a Subaru represntative took it for a spin and uploaded some new software to the ECU, he also told me that I had an after market air filter fitted, this is not the cause but might not help things, but he sent me on my way to try again.

CEL gone but still lumpy.

CEL came on again back into Subaru. This time they changed the Air filter, Air Flow meter and joints in the exhaust(had something to do with maybe affecting the O2 sensor??)

So far, no Juddering and no CEL!!! Yippee!!300kms gone!!
I think that it may of been the Air Flow meter that made the difference, well thats my conclusion anyway
Subaru were very helpfull indeed.

Back to her good old self!

 

What make was the "after market" filter you had? Filters with large amounts of oil impregnation are known to do the MAF sensor no good.

The sensor used on the 99/00 cars seems to be more fragile than the older ones and is particularly sensitive to oil contamination.

 

Not to sure, but it was green filter with metal gauze, and was oil inpregnated. Cant see a name on it though?