My son has a 2000 S40. He has a P0171 code. The guys at the store said a Mass Airflow Sensor. So we replaced it, still has the code. The problem is, when the temp drops below 35 deg. it won't start. New sensor, still won't start. Anybody have any ideas.

 

Code is telling you the mixture is rich. Could be a fuel pressure sensor, intake air leak etc. did you inspect for vacuum lines or cracks in any intake tubing? You may also want to check fuel pressure at the rail. Does it start if you put the old MAF back in? Also the cold no start makes me wonder about the engine coolant temp sensor... you can measure impedance to check (although the connector is an odd slide in clip that kind of works backwards...). How many miles on the car? While the MAF is one possible cause, there's other thinks to check first (hopefully you didn't throw out the original).

 

What fixed it? Inquiring minds want to know.