My money is on the fuel pressure sensor. I had one go bad and its fairly common on this car. It will either cause a too lean or too rich condition and may cause the car to hesitate.i got the check engine light come on as well and used an OBDII reader. got the P0172 system too rich as well as the P0101 mass or volume air flow circuit range/performance msgs.
would cleaning/replacing the MAF fix both msgs? i am thinking the too rich might be a byproduct of improper air reading.
any suggestions guys?
i forgot to mention, the car is all stock, it's an 05 S40 and it has 152,000 Km i picked it up used a few months ago but didn't get any service records so i am not sure how old the sensors might be and if they had been changed in the past.