Q: Recurring Check Engine Light on 2001 Volvo S40

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I have a 2001 s40 that I have been driving for the last two years with minimal problems. Recently I got a check engine light and noticed the car was not running that well - idle at really low RPM, poor pick up. I took it to the mechanic, he read a p0172 and decided to change the engine thermostat. I drove the car for about ten miles and the check engine light returned, this time it read a p0101. The mechanic told me I could continue to drive it for a couple days before bringing it back to him. As I drove the car the check engine light would go away for a bit and then come back, this cycle continued a couple times. After having the mechanic take a look at it again he said he needed to clean the MAF and everything should be fine. I drove it out of the mechanics shop and the acceleration was much better. Five miles down the road I got the check engine light again and the code reading p0163. I drove it for a couple days during which I was getting poor gas mileage, rough idling, car stalled a couple times, and acceleration was still not "normal". This time he changed the MAP sensor. I drove the car around for a about 50 miles and then started to notice an occasional rough idle, and it stalled once - but no check engine light until a few miles after these symptoms kicked in. Anyone have any clue what may be going on here or what needs to be changed or looked at? I have a smog check for renewal coming up so I need to figure out what is going on here. I am debating taking it to the dealer and biting the bullet, but I wanted to see what your thoughts were.

I greatly appreciate any help/advice you guys might have.

(4) Answers
(1) Comments
I have a car that is the same year, make, and model and am having the exact same problem. Did you ever get this issue resolved? If so, what was the issue?
I have a 2002 s40. I have engine code p0326 and p0101.

MY car stalls at least three times a day. Running a bit rough now and not accelerating as good.

Ive changed in the last year, 2 maps, fuel reg, 2 o2 sensors, spark plugs, coil kit, MAF, fuel filter, little hoses, etc.
1. Check for intake air leaks, especially in the hose going from the Mass Air Flow (MAF) sensor to the turbo inlet.

2. Check the boost pressure sensor with a scan tool while driving under a load. Boost should be about 13 PSI.

3. Check the boost pressure control system. Check the vacuum hoses from the intake manifold to the boost control valve, and the hose going to the turbo waste gate. 4. check the intake manifold gasket for leaks , expecially when cold, because as the engine warms up te gasket expands and actually seals the leak
Well considering you have a smog check, it's always good to get it checked out at the dealership. The check engine light (MIL) didn't turn on right away because it wants to see the problem ocurr twice. It's known as a "2 trip code", each trip being a time that the symptom ocurred. Since i can't properly diagnose it from where I am, i can only give general hints. It sounds like a possible fuel problem. Quite possibly too high of fuel pressure if it only runs rough at idle. If that's the case, then it could be a bad fuel pressure regulator. Once again, get it checked out by a dealer, yet feel free to point them in the right direction so they don't get confused and start throwing parts at it. I've seen technicians do that and get burned badly by it.

Hope things go good with the smog check for you!
I agree with both previous answers, and would just like to add my $.02.
These S40's had some issues with the fuel pressure regulators and the idle air control valves, so these should be checked.
Taking it do the dealer will generally ensure that the mechanic has experience with your specific vehicle, and possibly your specific problem. Try to ask for the shop foreman or smog technician to work on your vehicle if possible.
While at the dealer, ask if they have the capability to check and update the engine control module software if needed, and have them check for any recalls on your vehicle.
Let us know what you end up finding out!
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