740il, O2 sensor constantly reads 0.4 volts even with new O2 replacements

[DrvBMW4Life]

Has anybody had this problem before? I have replaced all sensors, taken it to the dealer and a local private shop, but no avail. This all happened when I replaced the stock catalytic converters to some aftermarkets a year or so ago.

Initially, I got codes indicating that one of the catalytic was bad. So, I replaced them both and now I continuously get the 0.401volt readings. Finally, I decided to take it to the dealer for a diagnostic check.

The dealer's (stealer's) top mechanic told me that the the factory engineer rep said that I should only use BMW catalytic converters because they are tuned with the exhaust and sensors. Nonetheless, even the shop mechanic had a hard time believing this. Although he used his shop diagnostic equipment, he could not find a problem with any of the other engine sensors nor with the ECM. He read ~ 0.4volts on the sensors as well. Since the vehicle is a 1996, the factory rep had no interest in looking any further or making anymore recommendations.

I even sent the ECM off to get tested...all tests came back good. Even though I told them that I was having problems with the O2 sensor voltage, they claimed that it tested fine for the O2 voltages and that nothing was wrong with the module.

I read somewhere on a forum that someone had used a spark plug oil adapter with the second (post)O2 sensor in it to slight delay it's response. He said that it worked long enough to get it inspected.

I would prefer to know exactly what's causing this issue. Please, any suggestions would be considered.

BTW - the engine runs unbelievably smooth in 'open loop mode'. I just want the engine light to remain 'off' and the O2 readings to change properly. I still kinda believe that it's the ECM. All O2 readings start out at 0.4volts without any fluctuation, but eventually goes to down to ~0.2 volts. Since installing the catalyic converter, the O2 reading has never gone above 0.475volts

[shogun]

I would check for broken wires from the Motronic to the O2 sensor.
Do not know the E38 that much, but on the E32 I can test the wires on the pins of the Motronic on top with running engine.
I posted that here with a pic
you can check from the top of the Motronic in e-box
But that is the pinout for E32, ask here on the E38 forum
The 7 Series E38 Forum