Idle Air Control Valve Fix

My 1999 Subaru Legacy Brighton (2.2L EJ22 engine) had an issue with its idle air control valve/solenoid. Over the course of many months, I got a handful of CELs (check engine lights) with a code of P1507: "Idle Air Control System Malfunction". The first few times I just cleared the codes, after all, the car was running and idling smoothly. Eventually though, I decided it was time to address the issue.Simply replacing the IAC valve is quite expensive: for a factory part I'd have paid $350 for the part alone. A local parts store would charge about $180 for an aftermarket version.Most typically, the IAC valve just gets too fouled up with gunk and needs a good cleaning. Here's how to go about it:

Locate the idle air control valve. On a '99 Legacy 2.2L, it is on the driver's side of the throttle body. The factory part will have a blue sticker on it giving the part number (A33-661 R02).
The valve is held on by two screws which have a reputation for mushing to pieces under the force of a poorly placed screwdriver, so make sure you have the right size (use a flat-head, not phillip's) make sure you have a very firm grip and plenty of pressure on the screw head.

I found all kinds of black gunk on the tip of the valve. Before cleaning it off, leave it plugged in to its wiring harness and lay it on the engine in a place where you'll be able to see it from the driver's seat. Turn the ignition key to the 'on' position (not start): the tip of the valve should move in or out, and back again when you turn the key off. If you don't see any movement, then it's likely your valve it either way too gummed up or, worse, the electrical/mechanical components inside have failed. Either way, you'll want to clean it up and try again.
Before cleaning, I removed the rubber O-ring from the valve (you don't want the cleaning agents to degrade the rubber). Next I used carburetor cleaner, rags and Q-tips to thoroughly remove all deposits from the valve. In the process, the spring and seating parts came out of the solenoid - I'm not sure if that is supposed to happen, but I was able to put them back in without problems.

With the valve removed, I also cleaned the air passages in the throttle body which also had plenty of black gunk.
My IAC valve was shiny-clean, so I replaced the O-ring and plugged it back in to the wiring harness to test it. I turned the ignition to the on position and it appeared to move in & out much more freely than before.

After ensuring that all parts were clean and dry, I re-installed the valve. Also, ensure that you have correctly replaced the air box, and that all other air intake components you may have removed are tightly reconnected.

Voila - it again idles smoothly.

Typically, an IAC valve goes bad by fouling, like mine did. However, they can also fail electrically or mechanically. Check for voltage in the wiring harness and continuity between the pins on the solenoid itself. I rule out mechanical failure by cleaning thoroughly and observing actuation of the solenoid when turning the ignition to the 'on' position.