94' Volvo 855T P0131 after a new O2

henrysin44 · Post by **henrysin44** » 24 Dec 2018, 09:41

I replaced my O2 sensor about 10,000mi ago, but recently the car started running pretty poor with bad MPG again. I unplugged the MAF sensor and the code went from P0131 to the absent MAF sensor code so I swapped MAF sensors with one from a junkyard and the car ran fine for 2 days with no codes but now the P0131 is back. I have a hard time believing the new O2 sensor is bad, anything I should look for? I've already checked for vacuum leaks and the wiring to the sensor looks fine.

Post by **abscate** » 24 Dec 2018, 10:08

Unless you removed and replaced every vacuum hose , don’t rule out a vacuum leak.

Smoke test is best, remove and replace takes longer but is sure to find and fix any leaks

henrysin44 · Post by **henrysin44** » 24 Dec 2018, 10:11

abscate wrote: ↑24 Dec 2018, 10:08 Unless you removed and replaced every vacuum hose , don’t rule out a vacuum leak.

Smoke test is best, remove and replace takes longer but is sure to find and fix any leaks

I actually did, elbows an all, even a new throttle body hose.

Post by **misha** » 24 Dec 2018, 12:26

What condition is flexy pipe?
It can disturb front o2 if it's cracked.

Generic o2 sensors will not work...usually.
You need oem Bosch with OE connector.

henrysin44 · Post by **henrysin44** » 24 Dec 2018, 12:55

misha wrote: ↑24 Dec 2018, 12:26 What condition is flexy pipe?
It can disturb front o2 if it's cracked.

Generic o2 sensors will not work...usually.
You need oem Bosch with OE connector.

The flex pipe is in pretty poor condition (missing the crosshatch cover) but i couldn't feel or hear any exhaust coming from it, the sensor is OE Bosch. When I view the freeze-frame data it says heated catalyst N/A, I don't know if that means O2 heater absent or no CAT. It is cat-less with a flashed ECU to ignore the rear O2.

Post by **misha** » 24 Dec 2018, 14:00

Rear o2 on '94?
Are you sure?

henrysin44 · Post by **henrysin44** » 26 Dec 2018, 10:51

misha wrote: ↑24 Dec 2018, 14:00 Rear o2 on '94?
Are you sure?

Yeah VIN and documentation says 94' but it has OBD I and II, and had a rear O2.

I'm now getting P0131 and P0172, the codes are pending codes none have set MIL to ON. But while driving ill randomly smell rich fuel and see my MPG drop but then go back to normal, it seems like a wiring issue and with the P0131 should I be looking into the wiring of the O2.

Post by **misha** » 26 Dec 2018, 11:04

P0172 is usually a vacuum leak.
Car is running rich.

henrysin44 · Post by **henrysin44** » 26 Dec 2018, 13:51

I don't want to jinx it but I think I've solved the problem, The O2 sensor connector had water and oil in it so I cleaned and dried it out, there was also an extension connector for the O2 so I removed that and plugged it straight into the main connector. After driving it a bit the codes went away.
If not then I suspect the intercooler could be leaking as everything that air passes through was replaced a month ago, I have a spare from a 2000.

henrysin44 · Post by **henrysin44** » 29 Dec 2018, 14:38

It rained a lot yesterday and I drove through minor flooding and now the P0131 and P0130 are back, I cleaned the sensor out again but this time added dielectric grease inside the connector, but the code comes back (as pending) after a few miles of driving. It runs fine, great throttle response, constant idle, and MPG around 30 according to the computer. But the second P0131 sets the MIL i get poor MPG and runs terrible.

I've checked for vacuum leaks but the car doesn't feel like it has one and I've never seen a vacuum leak throw a P0131.