New OEM Bosch Rear Oxygen Sensor Showing Low Volts

[Groosh]

Hello. I'm new to the forum and excited to have our 2009 C30 with 100K miles. When I bought the car, the PCV diaphragm was ripped and a lean code was coming up on the scanner. I replaced that while barely driving the car with the dipstick pulled. That is all fixed. But now, I got a 400D code signal too low for the rear bank oxygen sensor.

I went to replace it with an OEM Bosch unit and found it was not even finger tight. I replaced it anyway, cleared the code and within five minutes I'm getting the code again. I undid the negative battery lead overnight, then tried again this morning and the code is back. It is running on average about -.75V.

Does anybody have any ideas what to try next?

As a side point, I was not able to crack the front, pre-cat, oxygen sensor loose. But there is not code for that one. I have a new unit to install if I can get the old one off.

Thank you, Paul

[Groosh]

Also found that "Leakage air through throttle blade" was running 4.60. Is this normal?

[RobsS40]

I don't know about your leakage / 4.60, but low O2 volts means lean. Do you have any other diag. codes?

Your front one could be bad, but I'd check for other problems. That error may mean you need to clean, or replace the throttle control. I'm guessing it's "drive by wire"? Either way, a good careful cleaning might fix it.

I had a tough time getting my front O2 out. O2 socket (has slot for wire) splayed out. Normal 24mm deep socket wouldn't work until I chopped off part of the sensor. Then I had to remove the battery and battery box to get wrench handle and extensions to where I could put torque on it. Thankfully it eventually came out. I use thread chaser, and extra high-temp anti-seize compound and it's working well.