I know this thread is old, but in case someone else comes along and finds it later like I did here's what worked worked for me. Car is 1998 Legacy GT 2.5 with 5 speed. Checked all the vacuum lines, which seemed OK, then replaced the IAC ($300) and NSS ($40), but CEL kept coming on, usually when going downhill, esp when hot outside. Checked the vacuum lines again and this time noted that even though they seemed OK initially that two of the lines going to the EGR vacuum modulator were actually hardened on one side from 20 years of engine heat. In fact I had to use a knife to scrape that side off the attachment nipples. Replaced all 3 of the smaller caliber vacuum lines ($4.50), and the CEL hasn't come on since. My theory is that the lines leaked going downhill due to maximum manifold vacuum at those times with the engine braking and letting off the gas pedal, and it happened when it was when hot outside because the soft side of the hoses at the nipples loosened enough with the extra heat to leak a little, esp with the hardened side being essentially glued to the nipple. So even though the hoses looked OK and seemed tight they were actually leaking in right (or wrong) circumstances.
Bottom line should have spent $4.50 to replace all the old vacuum lines (even though they seemed OK) before doing anything else.
