O2sensor problem

Hello all, I have a 1993 e300 3.2L auto with HFM. and 220KM.The car runs quite well other than I keet getting a code 9 for a faulty O2 sensor. It uses a 4 wire sensor. I replaced the O2 sensor with a compatible O2 sensor not an OEM and I still get the code 9 for a faulty O2 sensor. I attached my meter to it while it's running and I get values between 0.1V -0.8V .It bounces quickly , unlike the old sensor which had little fluctuation and the readings mostly right around 0.4V. I'm considering pulling the connector at the ECU and checking for a wiring fault or corrosion.

Does anyone have any other ideas where to look to fix this?

Well as it turns out there has been no O2 sensor code in over 3 weeks. Initially it threw the code but now it's fine. Maybe it had to dial itself in??!! So far the car is throwing no codes..woot!

Did you reset the ECU/PCU, after the H20 sensor replacement?
If not, it had to go through a drive cycle 1st befor clearing itself.

I built the 3 pin LED box to check codes and followed the instructions. After a code is presented hold down the switch for 6 or so seconds and release then do the same for any other code. I checked for any codes and none came up on the LED.

Thanks for the reply..now it's on to fixing a new to me ML320 woo pee