94' Volvo 855T P0131 after a new O2

Post by **misha** » 29 Dec 2018, 15:38

Try to re-seat ecu...or check for corossion on contacts.
Since cleaning and re-seating 02 sensor's connector cured the problem for a while...it seems that you are on the right track.
I would check o2 connector from the wiring harness side...and try to minimize the gap between the pins with a small precision flat screwdriver.

henrysin44 · Post by **henrysin44** » 30 Dec 2018, 20:33

Well the wiring seems fine, picked up some carb cleaner to see if I might have a hidden vacuum leak somewhere and found one at the base of the vacuum tree and the brake booster hose(fixed), carb cleaner managed to stall the engine finding it. I'm going to order a new tree seal from a local place tomorrow but I still find it hard to believe a vacuum leak would give a P0131.

henrysin44 · Post by **henrysin44** » 30 Dec 2018, 21:45

One thing I forgot to mention is the car runs terrible until warmed up, idle drops and recovers, sluggish and feels like it will stall out, once warmed up it runs great. This would happen before the car started acting up and tossing a P0131. Wondering if maybe the coolant temperature sensor would do any of that and be a contributing cause to the P0131.

I've been searching the net about this problem and found an old forum post I made here and this was happening with the old sensor, replacing it solved the problem.

Post by **misha** » 31 Dec 2018, 05:06

Usually...ECT will throw it's own dtc...not o2 sensor dtc p0131.
Since the sensor is oem Bosch,i would say that wiring harness side of connector is bad...especially since the code is intermittent.
Clearing the gaps on connector with precision small flat screwdriver might be the best solution.

O2 sensor fault will not participate in idle problems you have.
Bouncing idle is more of a bad MAF problem or it's connector since they are prone to failure..BUT that doesn't fit in o2 sensor dtc.
You mentioned that you have flashed ecu installed...maybe it's at fault.
Stock ecu would be the best start...in order to rule out the flashed one.

henrysin44 · Post by **henrysin44** » 31 Dec 2018, 08:01

I'll give the gap clearing a try, I've inspected all wiring in the engine bay and tested continuity from the O2 all the way to the ECU and everything checks out. I did put the original M4.3 back in and it had no change, P0131 would still come up. Oh and the code isn't intermittent anymore, comes up 2 minutes after clearing. Unless a faulty MAF sensor could do this I can't find anything that could cause the O2 sensor to do this.

Post by **misha** » 31 Dec 2018, 08:25

You would get a maf dtc....but try to reseat it's connector few times and also check gaps on it's pins too.
I had a bad maf connector(gap between pins) which i fixed with small precision screwdriver...tapping them closer to each other the best i could.

henrysin44 · Post by **henrysin44** » 31 Dec 2018, 16:25

misha wrote: ↑31 Dec 2018, 08:25 You would get a maf dtc....but try to reseat it's connector few times and also check gaps on it's pins too.
I had a bad maf connector(gap between pins) which i fixed with small precision screwdriver...tapping them closer to each other the best i could.

I tightened the gaps between the pins of the MAF, O2, and ECU connector. Still no change, i'm still waiting on the vacuum tree O-ring but i'm not too hopeful it will clear the code.

I'm also still getting long starts, and running terrible when cold started, feels like the car will stall out and no power until operating temp....I feel like its connected to whats causing the O2 sensor code

Sommerfeldt · Post by **Sommerfeldt** » 02 Jan 2019, 05:29

Remember that the IC is "supposed" to leak air out through the bottom. If you have other leaks in it, they're usually quite easy to find. A smoke test is also pretty simple to set up if you're only testing the IC, but plug the hole in the bottom before you blow anything into it.

You said you have a rear O2 sensor - have you replaced both of them, then, or just the one? Your symptoms sound like a shot MAF and O2 sensor combined to me, but the coolant temp sensor can definitely contribute. If the ECU thinks the engine is at a different temp than it really is (in your case, thinking it is already warm, it seems), it will make terrible decisions, not put on socks and run around without a good jacket.

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henrysin44 · Post by **henrysin44** » 07 Jan 2019, 15:59

I replaced the vacuum tree gasket and the car is now vacuum leak free, but the code still comes back, although the car drives fine when the code is set now. However over the summer I cracked the thermostat housing (bottom) by over-tightening the top housing bolts, I replaced the whole housing with one from a 2000 V70R, my 850s ECT sensor wouldn't fit in it so I wired up the larger thread V70 sensor to it.
I found the ohm values are different with the V70 compared to the 850 although the gauge rests at 3 o'clock where it should. Worth looking into?

850 V.S V70 ECT values: viewtopic.php?t=79330

Sommerfeldt · Post by **Sommerfeldt** » 08 Jan 2019, 02:42

Yes, it is worth looking into.
If your V70 ECT is telling the ECU 300 ohm, the ECU thinks the engine is at 80 degrees, even though the ECT is saying it's at 40.
They're like a Swede and Norwegian talking to eachother. The Norwegian says something is "rolig". The Swede thinks something's funny, but the Norwegian is saying something's calm.

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94' Volvo 855T P0131 after a new O2

Re: 94' Volvo 855T P0131 after a new O2