p0138 o2 Sensor fault code help please
 

Hi all,

I'm stuck with my hands in my hair and don't know what to do. I love Volvo but this issue drives me crazy. So let me explain. I bought the car 2nd used and the prev owner told me the cat got stolen and insurance company covered it so they installed new cat and new o2 sensors. Well now i got a p0138 code saying rear o2 sensor high voltage. But when i look trough my obd reader it also shows sensor 1 giving high voltage but no error code. It's fluctuating between 0.02v and 1.275v. but mostly sits around 1.275v for a couple seconds. both sensors do this and i don't know how to solve this.

What I have tried already:

Changed both o2 sensors for OEM quality ones but code came back.
threw in a catalytic converter cleaner but didn't help.

What i do notice:

On idle im closed loop. When i give some gas and let go it goes into open loop. ( Is that normal idk? ) but when i go full throttle after like 4k rpm it goes into open loop as well. is that even normal?

2nd thing what i noticed is that when i let go of gas on the highway going 60/70mph down to lets say 20/30 the voltage stays very high of the 1st sensor and goes into open loop so i don't know if that is maybe a cause of the issue?

I have checked under the car where the cables are of the o2's and they look good and not bitten or anything by a rat. I really badly need help with this. I have heard a lot of Volvo's s40 have this issue as well.

It's a volvo s40 2004 mk1 *1996-2004* 2.0i B4204S2.

PS if needed i can upload a video of voltages of my obd2 reader. I'm just not on phone atm writing this from pc.

Firstly I guess your catalytic converter and 2 lambda sensors are non volvo ?? you cant guarantee anything unless they are genuine volvo parts.

How do you know you have you got the right sensor in the right hole ? They are not the same..

Well, it was done by a mechanic because it was covered by insurance company, 2nd thing. As far as i know the first sensor the one before cat is the up stream, the one without holes. The 2nd sensor is downstream sensor and i've used Denso. OEM quality is what it said.

So.. back with another clue. It happens a lot of times after i wipe the code it comes back when braking all the way down to 0 from 60mph.. Also noticed starting gets really difficult when pressing the brake pedal. Also i hear a squaking noise when pressing the brake pedal... I'm guessing the brake booster(Line) has a vacuum leak? Anyone ever had this issue? And if so do you have tips or tricks to how to fix it?

The difference in upstream and downstream cable lengths usually denote where they go.

If it helps I renewed the whole exhaust system, cat through to back box on my 2003 S40 fitted Lucas o2s's. I kept getting random codes come up that I've never known before, running lean this and that, changed fuel filter from original, cleared codes relearned the ECU and that sorted any reoccuring lean/rich codes. Just left with an errant P0340 codes
This is down to a failing voltage regulater on my alternater, signal noise disruption can throw a code or two that has no relation to code given. (Eliminated all Cam P sensor possibilities)

Hope this helps anyone.