I have an MY98 wagon with approx 54k.

A while ago I noticed that the car was struggling to idle after motorway driving. It would settle down after a couple of minutes but this concerned me. The car needed a service so I expected this to improve when done.

The problem has actually got worse. I contacted my dealer who informed me that it was a problem with the Idle stabilisation control. This was replaced, but the problem continued.
The dealer then tried replacing the Oxygen sensor, but this failed to improve things.
Although, since the ISC was replaced it has improved slightly, the problem is still there.
My dealer is currently contacting Subaru technical, but I thought I'd also ask the question on here to see if anyone else had experienced this problem?

 

I had a problem when my cat starting breaking down.

Driving would be fine but idle would hunt between stalling and 1500 rpm. I had to keep on the gas at every junction, very annoying.

Apparently the ecu just couldn't work out how much fuel/revs to give as the cat gradually broke down.

Maybe...

 

That sounds identical to my problem.

Thanks.

 

It can be the throttle posision sensor.