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Doug C.
Doug C., ASE Certified Technician
Category: Volvo
Satisfied Customers: 5643
Experience:  Volvo Enthusiast and extended work experience from a Volvo specialized shop
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Volvo S40: I replaced the MAP. I tried it in both positions

Resolved Question:

I replaced the MAP. I tried it in both positions but it did not turn off the check engine light nor clear the code. I guess both could have been bad at the same time, but unlikely. Do i need to replace the MAF sensor now?
Submitted: 4 months ago.
Category: Volvo
Expert:  Technician Chris replied 4 months ago.


-What year is your Volvo?
-What make and model is your Volvo?
-How many miles are on your Volvo?
-What type of engine is in your Volvo?
-What have you already tried?

-What codes are in the system? Please list each one.


Customer: replied 4 months ago.

02 S40 1.9 turbo. 108K miles. I got a code PO107 and a PO101. I replaced the MAP sensor. I bought one new one and tried it in both locations and still get the same code. Is replacing the MAF sensor the next logical step?

Customer: replied 4 months ago.
Relist: Other.
I am not getting any answers in a timely manner. I waited for more than 30 minutes the first time and all he wanted was mileage on the car. Who cares? Now I've waited for another 30 minutes and still no answer.
Expert:  Doug C. replied 4 months ago.

I'm sorry you are having difficulty getting a response, I'll be happy to help you out.

I do need to know what exactly the back story is on the situation though. You mentioned replacing the MAP sensor, I'm assuming this means one of the boost sensors (either on the intercooler or on the manifold)?
Was this done due to an error code, and if so which one?
Is there any drivability issue currently/
Customer: replied 4 months ago.

I don't know what more to say than what I've already written. To sum up once again. Check engine light is on. OBD reader says PO107 as the main fault with Po101 as potential faults. Yes, replaced the boost sensors. I bought one new one. I tried it on the intercooler first. No fix. I tried it on the manifold next. No change. Still have check engine light and still get the same readings on the computer. Wife says the car "hesitates" when she tries to turn it over. I noticed it died out once at idle when I replaced the boost sensor.

Expert:  Doug C. replied 4 months ago.

I needed more information like that... I don't know that it was a P0107 fault or that it hesitates etc unless you tell me.

When you clear the faults, do you see the P0101 occur first (if you scan right after the light turns on) and then the P0107 if you continue driving?
Customer: replied 4 months ago.

No, faults never cleared. Using CAR MD which is a piece of junk but it is what I bought. It won't clear faults it just reads what the computer tells it. It has PO107 as the "failure code" and P0101 as "Potential Failure" Codes. the CAR MD trouble shooting says change the boost sensor. Unfortunately, if that doesn't fix it, CAR MD can't tell you anything else. There is no way to continue to trouble shoot. I read about the MAF by reading stuff on-line. It did not change after I replaced the boost sensor. I took a second reading after putting the new boost sensor in the intercooler and got the same exact reading as the initial. I swapped the boost sensor to the manifold location and got the same exact reading.

Expert:  Doug C. replied 4 months ago.

That complicates things considerably.... neither one of these faults (101 or 107) will just go out immediately.
A boost sensor fault can take weeks before it clears; a MAF fault often at least a dozen successful tests.

Reading the data values is only moderately helpful here due to the way the system works. Give me a little bit to type this out, it is a bit lengthy.... I know you've been waiting a while and I don't want you to think I abandoned you.
Customer: replied 4 months ago.

I had to go to church anyway. I'll wait for you to reply thanks for sticking with it.

Expert:  Doug C. replied 4 months ago.
Thanks for your patience.

Just to touch on the hesitation starting before getting into it, note that this would be unrelated to the fault codes. These codes will not cause any hard starting at all, at worst just poor acceleration from a stop, coughing, etc. If you are having a hard time starting it, the majority of the time this will be a leaky fuel pressure regulator; another super common failure on this model.

The air monitoring on this vehicle is a bit unique compared to most cars as the MAF is used as a test device against the two boost sensors, and vice versa. Because of this any failure on either can cause a fault on the other... sometimes solely on the other.

It is extremely often that we will see a P0101 (MAF sensor) and the only fault, but the failure point is actually boost #1 (manifold) sensor.

On a normal car the MAF sensor reads the volume of air coming in and the computer uses this to determine the fueling amount as it reads the air volume.
This occurs on the S40, however it also uses the dual boost sensors to calculate air volume actually inside the manifold/piping.
By reading the pressure from boost sensor 2 on the intercooler and comparing it against the atmospheric reading on boost sensor 1 on the manifold, it can calculate the approximate air volume inside the manifold and piping. If it knows the atmospheric pressure, and it knows the compressed pressure, it can determine how much air volume would fit at that compression reading, at that altitude (atmospheric reading).

This is a bit unique of a situation in itself.... most turbocharged vehicles will just have a single MAP sensor... but on top of that, this calculation is then cross checked against the MAF reading to make sure the air volume entering corresponds with the air volume observed.

The result here is that you can have one boost sensor skewing slightly which will cause an erroneous volume calculation that will cross check against the MAF incorrectly. The go-to response for the computer when this occurs is P0101, MAF fault. This is because the computer is not smart enough to realize one of the boost sensors failed unless it reads a completely implausible reading (40 psi at idle for example). If it is just skewed (reading 8 inHg instead of 17inHg for example), it will assume it is a valid number since it is plausible, then it will fail on the cross check with the MAF.... and again, the go-to response here is a MAF failure, even though in description we can clearly understand it is not.

The reverse can hold true as well, that a MAF reading error can cause erroneous boost sensor faults, it is extremely rare though.

It sounds like you've been more or less on the right path here. The odds of it not being a boost sensor are pretty low, however we wouldn't know if you fixed it unless you cleared the faults and they did not return (boost sensor faults normally return the second or third time you drive the car).
Your P0107 again was for the intercooler boost sensor, however remember that a skewing of sensor readings can/will result in trouble codes... meaning that you may have a boost #2 that is working perfectly and boost #1 is reading incorrectly causing #2 to appear wrong. Yes, it's a mess of a system.

The best thing to do here first before even buying anything is to check for vacuum leaks throughout.... this means injector seals, manifold gaskets, intercooler piping/couplers, and especially the idle hose between the throttle and the intercooler. If you have a vacuum leak affecting your pressure readings, you will get erroneous fault codes.

If no areas of concern are found, take the two boost sensors and reverse them, then clear the faults... the majority of the time you will find either the situation corrected, the situation significantly changed (drivability betters/worsens), or a change in what codes you see. These are all tell tale signs that you have one boost sensor failed. At that point it is ideal to replace both (they both are just as old as each other), however you can try one as you've done and re-evaluate.
With a new sensor in the intercooler (again since that is the fault being thrown), you would clear the fault and drive the car.... if the vehicle does not fall on it's face on throttle from a stop, and/or the codes do not return, you should be good.
If it stumbles badly on throttle and/or the codes return (same or different), you would swap the new sensor to boost #1 and reinstall the original boost #2 on the intercooler.

If there is still no change, then and only then would we consider the MAF as a plausible cause. It is always the least likely scenario especially if you are seeing boost sensor faults too. At that point you can remove the MAF and try some MAF cleaner or brake cleaner to clean it out, let it dry, and retry..... it is usually beyond that if it is failed though.

I can tell you from years of working on the S40/V40 I have seen maybe one or two MAF failures, but I have had to have changed at least a hundred or more boost sensors. They are a known weak point as well as a very difficult failure to diagnose if you aren't familiar with the backwards system they use.

Before doing anything else, however, you need to clear your faults and see what returns. Waiting for the light to go out can take a very long time, whereas the fault returning if the problem isn't fixed will only take minutes (a couple < 1 mile drives).
Since you already have the one sensor, put it back in the #2 position and have the fault cleared and see whats up... if it returns, swap it to #1 position (remembering to put the original #2 back in place not move the #1 there since we wouldn't be eliminating it that way), then clear the faults and try again. If you have identical behavior/codes in both situations and you have no vacuum leaks, then and only then would you consider a MAF as a possibility.
Doug C., ASE Certified Technician
Category: Volvo
Satisfied Customers: 5643
Doug C. and 5 other Volvo Specialists are ready to help you
Customer: replied 4 months ago.

Since my CAR MD won't clear the fault to reset the system. Is there another way to do it?

Expert:  Doug C. replied 4 months ago.
Sure, you have two options.

If you live anywhere other than California, you can take the car to most auto parts stores (big chains like Autozone, Advanced, OReillys, etc) and they will clear it for free. California has tighter restrictions on this, and independent/privately owned parts stores don't do this. The big national chains just about all do however.

You can unhook the negative battery terminal for 60 seconds. Note that if you do this and you have the original Volvo radio you will need your radio code to make the radio work again afterward. This is usually included in your owners manual on a small card, however if you do not have the card (or otherwise know the number) you will need to either take the car to Volvo to be unlocked, or you can remove the radio to get the serial number and call Volvo with that and the VIN and they can get your a code as well.
Of course if you have an aftermarket radio this is not an issue.

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