Rich limit:
-Contact resistance in the terminals. Which terminals? The injectors?

As far as I know this is the resistance between the connectors for the 02 sensor. You also check to make sure they haven't really oxidized (rusted). Being on the underside they tend to corrode at a much faster pace. You can run an Ohmmeter between the connector there and up in the engine bay, but its unlikely the connection is severed (though if it were, its likely more codes have shown up because the 02 sensor wouldn't work at all...). Taking a good look at the connector and possibly cleaning it is all you have to do here.


-High fuel pressure. I can try to check, my mechanical gauge only goes to 30 psi. Don't I need somewhere around 65 psi?

Yes, I know its above 30, but I think if its in the 60's its too high.

-Leaking injectors. I guess I can pull them. I think the proceedure is to pull them, lay them on a newspaper or something, activate the fuel pump and see if they drip. Is that about it?

That's the way I've always done it, so that's what I'd do. However, there is a short-cut. Pull all your spark plugs, and put some tape or something so you know which cylinder they came from. If one is different from all the rest, there's your likely suspect.



-Contaminated oil. Just changed the oil on Friday, CEL came on on Sunday. It looked fine, no smell, nothing unusual. About what I'd expect at a 5,000 mile change. I guess that puts it in the unlikely column.

Yep, changing oil shouldn't have caused anything, but its one of those things that I'd reset the code and drive till it came on again.

-Defective heated oxygen sensor (HO2S). Is there a test for this? I'll have to search the web. I don't have a repair manual for this car yet (haven't needed one and the only Haynes I could find was only for European models).


It might be possible that there is some caked on crap covering them that might be removed if you use a propane torch to get the sensor just red (not bright red) which might help.

I don't know specifics on the s40's test but the basic procedure but look here for the method: http://www.brickboard.com/AWD/index.htm?id=1323596&show_all=1
That is a generic method to test Bosch O2 sensors, yours should be very similar.


Lean limit:
-Contact resistance in the terminals. Again, which terminals? Injectors?

-Low fuel pressure. See above.

-Air leakage in the intake system. I'll look today for broken, dried vacuum lines.

You can also try running along the lines with a propane torch that is just running the gas (so flame is off!). If some gets sucked in your engine will jump in RPM and you can figure out where the leak is coming from. If you had the ability to do a smoke test, that would be better, but not many people have that at their homes...

-Leakage in the exhaust system. Interesting. The flex pipe does need replacing due to a small leak. But the cracked pipe is downstream of any sensor so I didn't think that could be it. Am I wrong?

It shouldn't be a problem since the O2 sensor is before it, though stranger things have happened. My thoughts are that a leak there could help pull more air through the sensor making it read a lower O2 concentration, thus adding a bit of fuel each stroke that could foul the sensor in the end. This is unlikely though, but possible.

-Defective heated oxygen sensor (HO2S). See above.

P0171 is rich, so you have to worry about those. Why do you say P0171 is rich when the definition says it's lean?

The information I gave you was for a DTC (diagnostic trouble code- volvo codes). On most of the Volvo's there's a conversion from the universal P-xxxx codes to the Volvo ones. P0171 is ECM-25 if you were to use a Volvo scan tool (VST) and read your codes. P0172, which is lean, is also ECM-25, so both are listed. If you had the VST you would get ECM-25 and either Rich or Lean and either Intermittent or Permanent. That would help tell you what the issue is, but these cars aren't quite perfect yet. Its nice that it tells you a code and its down to 5 causes. When you get 2-3 codes it helps narrow it down, usually to just one..

Also, its possible for these two to be interconnected, which is why I left them there. These two are insanely annoying codes because they are quite difficult to track down. I could tell you its not an air leak, but it could possibly be. At 150k, I don't know if you've replaced your lines and an air leak could cause it to run lean long enough to foul the O2 sensor so now its running rich.. Now here's where I always get confused, but I think I've got it: When the car has your code P0171, it thinks its always running rich, so it leans out the fuel. If it has the P0172 it thinks its running lean, but it adds fuel so it very well might be running rich. Right now your car thinks its running rich and therefore is using less fuel than it thinks is proper. You crossed the threshold so the code went off. You might have been flirting with the other side where it was running lean, but not lean enough to set off a code. The car will continue to enrich the mixture ever so much resulting in unburnt gasoline hitting the O2 sensor. That cakes it and renders it useless.

The above just shows that it could be either rich or lean being your culprit, which makes this fix so annoying..



Now first off, has it come up more than once? One time back in April / May she started the car and it kind of sputtered for a few seconds and then ran normal. The CEL came on after a few minutes and the car was still running fine. Two days later the CEL went off on it's own and never came back on until now.

Did you check the code for this or did it vanish before you had the opportunity?

Yesterday I cleared the code (according to my Scangauge II it was cleared) but the CEL did not go out. I think it should have. I checked for codes and it read P0171 again. I was thinking of disconnecting the battery for an hour or so to see if that would shut the CEL off. Is that a good, bad or indifferent idea?

I believe this is an indifferent idea, though it might reset the fuel trim levels making it a possible idea. I think the CEL light came back on because the trim levels show that it was high. You need to drive 20-30 miles to get the levels down and then resetting the code should erase it if its not a permanent fault. If it is permanent the fault will come back almost immediately. I don't know for a fact if the will reset itself and run a diagnostic giving you a hundred miles or so before it would come back on.