Hey all. I just got done replacing both O2 sensors on my 05 WRX STI. It was throwing a running rich code, and getting abysmal fuel mileage, hence the sensor replacement. Now after the sensor swap, it hasn't  thrown the rich mixture code with a 5 mile test drive. Now however it has a wandering idle. If you disengage the clutch and let it drop to idle, it will either drop so low it actually dies, or it will drop to around 200 RPM before climbing back to 800. It will also occasionally speed up the idle to 1000 RPM. It had this wandering issue before the sensor swap, but it was never bad enough to kill the car and generally wandered up instead of down. I have only had this car a few months and the previous owner tuned it.

How is the idle controlled on this car? Is it all ECU controlled? Or is there something I can adjust? Do I just need to give the ECU time figure out it has new sensors?

Thanks in advance.

If this is a manual trans, I would suspect the neutral switch ..... YES Subaru stick shifts have a neutral switch. It tells the ECU to control the idle. I have had this on 3 previous Subarus with manual trans.

34 minutes ago, Ferret54 said:

If this is a manual trans, I would suspect the neutral switch ..... YES Subaru stick shifts have a neutral switch. It tells the ECU to control the idle. I have had this on 3 previous Subarus with manual trans.

It does it even when in gear with the clutch in. The neutral switch shouldn't affect it then I would think.

Clean your throttle body. As in remove it and scrub it out. With DBW throttle control the ECU uses the throttle motor for idle control. They get dirty, respond slowly, and they drop outside the range of the proportional, integral, and derivative (PID) gain controls the ECU uses to stabilize it's operation. 

GD