Hard start, engine light on after replacing catalytic converter

Hi,

Very stuck here! I have a new catalytic converter and have four new oxygen sensors. No engine light.

However, the car is starting rough and shuddering. When I connect my OBD2 Tool it is telling me that O2B1S2 (V) = 0.000 all the time, regardless of the car being on, off, accelerating, decelerating, driving or idle.

I have connected a multimeter across the two signal wires going into the ECM, and have used a test light to bridge the signal (not power) pins on the harness side connector. I would imagine this means the wiring in between the ECM and harness side connector is fine?
Have I got this right?

If not I assume there must be an internal problem in the ECU. Because I connected the multimeter to the signal wires near the ECU it gave a reading, meaning the ECU wires are connected inside.
For some reason the computer isn't reading the voltage?

Here is the weird bit:
Even when there is not a sensor plugged into the other sensor 2, when the car is running it reads V = 0.440 always.

Bank 1 Sensor 2 - no voltage. Wires are fine.

Is this a tuning problem?
Is this a wiring problem?
Is this an internal ECU problem?

Please help!

Update:

I tested the ECU wires (black and yellow) on the harness connector pins on both downstream O2 sensors.

The working O2 sensor gave a reading of 0.060 v
The not working sensor gave a reading of 0.000 v.

I have noticed the car starts rough every second time. I have consistently tested this, and it starts perfectly if you start the car, turn it off and start it again.
If you start it again, it rough starts. Turn off and on and it starts fine.

Is this due to the non functioning downstream sensor?


02B1S2 V = 0.000
O2B2S2 V = 0.885
EQ_RAT11 = 1.696
O2S11 (mA) = 1.008
EQ_RAT21 = 1.870
O2S21 (mA) = 1.129


Replaced bad catalytic converter in car listed above E90 N46 and replaced Bank 1 Sensor 2 and Bank 2 Sensor 2 (left top two Sensor 1's in).

This is causing a hard start up on the car and an engine light.

Unfortunately the O2B1S2 is showing V= 0.000. I would guess that this should be relatively close to O2B2S2?


I am getting fault codes for Bank 1 Sensor 1 and Bank 2 Sensor 1 (top 2), but not for Bank 1 Sensor 2 (V = 0.000).

Also, when I unplug Bank 2 Sensor 2 (V = 0.885) and start the car, it displays as V= 0.440, and V = 0.000 when the car is off. Why is this getting a reading when it is unplugged?
Bank 1 Sensor 2 remains the same V=0.000 all the way through.


So, my questions are:

Why would Bank 1 Sensor 2 be showing V = 0.000 all the time (car on or off)?

Why would Bank 2 Sensor 2 be showing a reading at all (V=0.440) even if it's not plugged in.

Are the readouts for O2S21 mA= 1.129 and O2S11 mA= 1.008 normal readouts?

Why would I be getting fault codes for only both Sensor 1's when Bank 1 Sensor 2 is always showing V=0.000.


I replaced the engine in my 2005 E90 N46 320i, and ever since a problem has developed which involves starting rough and almost stalling.

HOWEVER if you turn the car off, and back on again it starts perfectly fine. Turn it off and on, rough start. Off and on, perfect start.

This goes in a cycle forever and is always consistent.

On my cheapie OBD2 scan tool it says one of the downstream O2 sensors isn't reporting any output (V = 0.000), which should be reporting V= 0.440 even if a sensor isn't plugged in during start up. I've been told downstream O2 sensors don't have any effect on vehicle start up as they only work when hot. The wiring and pins have been confirmed to be working on the sensor and harness side connector, so it isn't a wiring issue. Must be something software related?

I DON'T HAVE AN ENGINE LIGHT OR ANY CODES. ALL SENSORS AND THE CATALYTIC CONVERTER HAVE BEEN REPLACED.

I've heard people struggle with hard start ups after replacing spark plugs and they needed an "adaptation tune" or something like that to fix the problem, but I've got no idea what this involves! Is this BMW specific, or can any auto tuner shop do this?

Any help is greatly appreciated. Thank you!