I have a Subaru Legacy BH EJ20 year 2002.
From the day I bought it, it had a check engine light (CE) with code P0519 - Idle Control System Malfunction.

The CE went away after a couple of drives, but it always came on while going down the hill with car in gear.

Idle was allways stable and there were no problems with the engine stalling. So I searched and searched for a solution. It was weird, because car was driving fine, only CE was illuminated.

After a while I read online that it might be a faulty neutral safety switch (32008-AA071).
So I decided to change it and from that point on the CE went away! Before car also felt like it had a lot of backlash in the drive train, after that it all went away.

It seems like the car allwas tought it was in neutral, so it wanted to set the idle RPM. But RPM wont fall when the car was in gear going down the hill, so it threw a CE with P0519 code.

I hope that this post is helpful.
Greetings from Slovenia!