Girlfriend has a 2001 Legacy GT, ran great for the last 2 years, more or less. Car sputters and stalls when cold out, after startup. Threw two codes, P0139 and P1507. First thought it was the MAF, but this engine has a MAP sensor, as far as I know. She says her mechanic went through it not too long ago, and they replaced the gas cap, lol. I don't want to replace the downstream O2 sensor, as I don't think that's the cause, but correct me if I'm wrong. Car has 183,000 miles on it, we're in upstate NY right now. Thanks in advance for your advice, never worked on Subarus before.

downstream (post cat conv) does nothing related to a:f or other parameters related to the engine's performancein that car (pretty sure only the H6es may use that sensor for a:f corrections). It is a 'nanny' for cat conv heating-up properly.

hopefully, someone with more experience will replay but one thing you might do is, in a new thread, ask for recommendation of a soob-friendly shop near your city. Someone may know a good independent mechanic.

Edited November 3, 20196 yr by 1 Lucky Texan

Try cleaning the idle air control valve (IACV). This could be causing issues with idle when cold. 

Not could be a number of other things too: 

- coolant temp sensor (*should* throw a check engine light) 

- dirty throttle position sensor contacts 

- dirty injectors 

Check the ECU for any stored codes. Find out what these relate to and investigate one by one. 

Fuel pump or dirty/poor fuel quality could cause issues too. 

Cheers 

Bennie