The ‘Service engine’ light came on in my meticulously maintained 1995 940. It would go off after a few miles and the engine was fully warmed. I checked the ECU and it blinked 2-1-2 for 'Faulty O2 sensor'.

“No problem.” I said to myself. I purchased a universal one and installed it, taking extra care to make it perfect. I fired it up, and held the button on the ECU for 5 seconds, just like the good books says, and the code now blinks 1-1-1 for the fuel and ignition.
However, the service engine light still comes on and stays on for a few miles, until the engine is fully warmed. It has not changed at all. I checked and cleared the codes from the ECU several times. Then I disconnected the battery for 30 minutes and reconnected it. The light STILL comes on.
What am I missing here, guys?

Thanks,
Jeremy
1995 940
1983 244