Replaced front O2, immediate misfires, high trim. Diagnostic help? Topic is solved

leapdragon · Post by **leapdragon** » 06 Apr 2025, 11:34

Hi all,

Just replaced the front O2 sensor on my '07 XC70 preventatively (old one was 230k miles old and I sometimes got the feeling it was a little lazy).

Replaced with Bosch 17137.

However, upon start car runs poorly, and I get p0101 MAF sensor value out of range or wrong, the long-term fuel trim is unavailable (limp mode?) and short-term fuel trim is pegged at max.

Obviously, something is very wrong... However, I can't see anything I'd have done that would create a new, massive vacuum leak. This suggests the new O2 sensor is not doing the right thing.

Faulty sensor? What else should I be looking? Did I overtighten it and kill it? I didn't torque it, just went for "pretty tight" (i.e. more than an oil pan, less than a strut mount).

Looks like for the time being I'll try to put the old one back (ugh, not fun, this is an irritating job) and see if that fixes things, but in the meantime, any help anyone can offer would be appreciated.

Vova585 · Post by **Vova585** » 06 Apr 2025, 11:44

I would run to the garbage can and extract the old sensor just in case. I am questioning if the new sensor was of the "proper" quality. Where you got it from? 131137 does not look like correct number. Fcp advertising
https://www.fcpeuro.com/products/volvo- ... ront-17137

leapdragon · Post by **leapdragon** » 06 Apr 2025, 11:55

I put the old one back and... same. High short-term fuel trims. Fuel is definitely off because now I'm getting multiple cylinder misfires with the old one back in, don't know if it's because I'm rich or lean, but short terms are pegged at 25.

So it's something else I've tweaked without meaning to as I replaced the sensor.

What are the other things in the area of the front O2 sensor connector that I may have screwed up? It's pretty tight in there, I've gone in with my arm through the wheel well doing it by feel to plug, unplug.

Thought I'd been pretty clean but apparently not.

leapdragon · Post by **leapdragon** » 06 Apr 2025, 12:00

Trying to get a clear view down into that area, there's a lot of wiring harness there being pulled in multiple directions. I am having serious groans wondering if I've created a wiring fault way down in that nest, on a harness

leapdragon · Post by **leapdragon** » 06 Apr 2025, 12:16

@vova, sorry, was Bosch 17137. The old an the new are pretty much identical.

leapdragon · Post by **leapdragon** » 06 Apr 2025, 13:48

- Switched back and forth again, both sensors behaving the same way
- Withdrew battery power for 15 minutes, reconnected battery
- Checked fuse
- Surveyed everywhere for any vacuum leak I could have caused
- No luck, car still stumbling, misfiring at idle, short-term trims still pegged at max

Played with the connector some more, I can't get anything to behave differently. Boy do I wish they'd put that connector in a different place...

I may be beaten on this one, I haven't been to a mechanic in over 10 years but when we start to get into serious diagnostics on harness, I may be out of my depth.

If anyone has anything to try / consider, thanks for any tips, otherwise looks like tomorrow I have it towed to the local Volvo shop.

Post by **vtl** » 06 Apr 2025, 16:24

Smoke it? You may have introduced a leak. Since you were in that area, I would check this hose https://www.fcpeuro.com/products/volvo- ... o-30680446 around PTC nipple and where it connects to turbo. Just pull it and inspect for tears. And smoke everything up well. You have a hole in intake, pretty sure about that.

I used 17137 in my 05, no problem.

leapdragon · Post by **leapdragon** » 06 Apr 2025, 16:56

It sure sounds like a vacuum leak, doesn't it?

Unfortunately I don't have a smoke machine. :-/

I've felt all the way up and down that entire pipe plus wiggled/checked all the connections coming off of it near the turbo. And the connection to the turbo itself. It's not definitive without smoke testing, but given that we're pegged at +25 short-term trim and potentially still running lean, wouldn't it have to be a pretty decent (and thus easy to spot) vacuum leak?

This feels very "hard to detect" for a leak big enough that short-terms are maxed yet we're still stumbling and misfiring on multiple cylinders...

Is it possible that the leak is at the manifold? i.e. that the pressure from untightening/tightening the sensor exacerbated an existing problem? (i.e. loose exhaust manifold, crack in downpipe, etc.)

I guess that still requires a smoke machine.

Vova585 · Post by **Vova585** » 06 Apr 2025, 18:37

If you want to try it yourself get this one.

https://www.amazon.com/dp/B0BSH2XS4R?li ... ics_f_asin

Post by **vtl** » 06 Apr 2025, 18:39

Vova585 wrote: ↑06 Apr 2025, 18:37 If you want to try it yourself get this one.

https://www.amazon.com/dp/B0BSH2XS4R?li ... ics_f_asin

$69 https://www.amazon.com/dp/B07JVT1NG2 I have this one, it works.