Subaru Repair: 1999 Legacy Check Engine Light, Subaru repair help subaru expert advice

Question
Check Engine Light Help, please! 1999 Legacy 2.2L 5spd 71,000miles, driven at 5000' altitude. I've owned it about 3 years with intermittent CEL issues. Dealer has read/cleared all codes. Recorded P1507 and replaced Neutral Switch at 55,000 miles Sept '06. PO325 at 59,000 miles and had Knock Sensor replaced. At 66,000 miles Dec. '07 rec'd PO1507 code and again replaced Neutral Switch. Since December, intermittent CEL light with same Neutral Switch code. Dealer has not been able to identify why this light continues to come on. Replaced gas cap - just in case and car continues to throw this code. Seems to come on at cold start for me, dealership had car for 3 days trying to replicate conditions - no luck. Dealership completed required Safety and Emissions and sent us home. Thank you in advance for any help with this issue.

Answer
Hey Kevin,

The P 1507 is a conditional code and very hard to accurately diagnose sometimes even by the most skilled technician.  A conditional code is set when the ECM (engine control module) recognizes a specific condition has been met for it to command on the check engine light.

P1507 is Idle speed control(fail safe).  What it means is that the ECM  doesn't like a particular item of data it is receiving with regards to the amount of control it must use to keep the engine idling at a specific range with respect to the amount of engine load it is under.  

We have replaced things from the mentioned neutral safety switch on the 5 speeds to weak alternators which is what I would suspect needs to be checked next.  The problem here is that most Technicians will do a quick check of the alternator and give it a pass/fail but we have found it necessary to run the vehicle with the hood shut and all accessories on for an extended period of time before condoning or condemning.  There are a lot of other possibilities but this is a common thing we have seen cause problems and I would start with a thorough test first.

Justin
http://allwheeldriveauto.com/