2016 XC90 P0171 - Out of Ideas

[coham]

Hey all - I've got a 2016 XC90 just shy of 80k miles. Threw a P0171 code (fuel trim too lean on bank 1) a couple months back. After doing some homework on this site and others, I've tried/crossed off:

• No squealing from the oil cap/PCV. Looks like the PCV might be leaking a very tiny amount of oil, but it seems incredibly minor. New one is on order anyway.

• Cleaning the MAF (looked brand new, but the cheapest/quickest thing to cross off)

• Smoke tested the intake. Supercharger seals were leaking pretty badly. Had the dealership replace them. No smoke leaks after the new seals.

• Replaced the downstream O2 sensor.

Dealership suggested the downstream O2 sensor as the cause when I brought it in for the seals, and recommended replacing both sensors. Replacing upstream is probably my next step.

What confuses me is that the car seems to be running rich. Gas mileage has dropped 2-3 MPG, it smells a little rich, and the tailpipe seems a little sooty. I read the new downstream O2 sensor voltage, and it seems to be around 0.88-0.9 V after idling a few minutes. My understanding is that 0.1 V would be lean, 0.45 V ideal, and 0.9 V rich. So if the sensor is indicating it's rich, I'm confused why it's setting P0171, indicating it's lean?

Curious if anyone else has hit this engine code and what fixed it for you, or if you have other suggestions to try. Thanks in advance.

[matthew1]

You've seen https://www.swedespeed.com/threads/2016 ... st-8167028 yea? Vacuum leak or O2 sensor, generally. Looks like your next step is the remaining O2 sensor... please let us know how it goes.

[coham]

Replaced the PCV today (sheared a bolt torquing it down - added a couple hours to the project while I ran out for an extractor), as well as the upstream O2 sensor. The washer gasket on what I think is the pressure valve on the PCV was cracked (looked like it might have been "stuck on" and cracked as I pulled it off), and there was a little rectangular plate with square cutouts sitting on the engine. I looked at the replacement and found the same piece inside the little rectangular hole (you can see the one that fell off in the fourth photo). Not sure what it's for, but it was broken.

I cleared the code, rolled it off the ramps, and let it idle. CEL was back on with P0171 within 2-3 minutes. I cleared it again and drove around the neighborhood with a live data view of the downstream sensor. It was solidly in the 0.8-0.9V range, and the light came back on within a few minutes.

Two things I did find interesting:
1) After the PCV/upstream O2 sensor replacement, the exhaust no longer smells rich.
2) The CEL is coming back on much faster than before. I cleared it after cleaning the MAF and having the supercharger seals replaced, and it took several weeks to come back on each time. Now it's back on in 2-3 minutes whether it's idling or being driven. I guess maybe it's good it's quickly reproducible?

I did see that post... I think I've crossed everything suggested on it except cracks in the exhaust piping/cat, fuel system stuff, and spark plug issues (though I haven't had any misfire codes, and the smoke test didn't show any leaks, at least). In another place I asked, someone asked about oil consumption and suggested piston rings. (It's an early 2016 model, with an engine serial before the rings were redesigned). I did get a low oil light about 5 months ago, but it was a couple days before I was planning to change oil anyway. It suggested I add a liter; I didn't measure what I drained, but it looked about normal. I checked the electronic dipstick and it's still fully at max despite changing it five months ago, so it doesn't seem to have an otherwise noticeable issue.

I think I'll poke around the spark plugs a bit, and then maybe make an appointment at a local independent Volvo place that people seem to like.

[coham]

Read https://www.swedespeed.com/threads/2016 ... st-8017552 which suggested resetting the ECU. Disconnected the main 12V battery in the trunk (along with the plastic module on top of the positive terminals that has a couple sensor wires, in addition to the sensor plugged into the negative terminal). Also disconnected the starter battery under the hood. (I don't know if it's necessary to disconnect the starter battery, but figured it couldn't hurt). Taped the two positive and negative leads off the main battery together and let it sit for a few hours. Checked the voltage potential across them before shorting them, and it was about 0.8V. (Don't do what I stupidly did and close the hatch after taping the battery leads together. I was wondering if there was any physical latch to get the hatch open with no battery connected as I was closing it. Turns out there's nothing very easy, and I had to crawl over the seats and sit on the pile of stuff that hasn't made it to Goodwill to reconnect).


Upon reconnecting, the downstream O2 sensor seems to be oscillating between 0.1-0.9V as expected, versus being pegged at 0.8-0.9V. I quickly got a red "12V Battery Charging Fault Stop Safely" dash error, which popped back up each time I stopped and started the car. I disconnected and reconnected the sensors around the battery (including what I think was the BMS connector on the top right), and also pulled fuse 51 from the under hood fuse box for a minute. This seemed to clear the error. I then got an orange "Start Stop Service Required" warning. I tried pulling the BMS sensor on the starter battery, and also disconnecting the starter battery with the BMS sensor (and the one below it?) disconnected, but I haven't managed to clear this warning (yet). One video online suggested disconnecting the main battery and BMS sensors while disconnecting the starter battery; I'll try this soon if the warning doesn't clear itself but would rather not see the "12V Battery Charging Fault" error again.

Idled and cruised the neighborhood for 15-20 minutes and was unable to get the CEL/P0171 to come back on when it was coming on within 2-3 minutes of idling or driving a couple days ago. The downstream O2 oscillated between seemingly sane values the whole time, eyeball averaging 0.45V. Hoping this might have fixed it, but I'll report back if not.

[Oly850]

I had a similar problem, same code on an S80 a few years back. What I learned is that the front/right O2 sensor reorts fuel rich / lean to the ECM, it does all the work. The rear/driver's side just reports on the efficiency of the catalyst.

I might just make darn sure you haven't crossed the connectors somehow (don't ask how I know this), replace both O2 sensors, clear codes, put a whole bottle of your fuel injector cleaner of choice in the tank and go drive it. It may take a number of drive cycles to clear the fault.