[Vanced]

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.