92 740 turbo: why check eng light?

Well, I went and bought a new NTK O2 sensor as I was getting a code
for a bad lambda. This would (or so I thought) set the CHK ENG lite
on for the first few minutes of operation, then it would go out and
remain out. However the code has gone away! Didn't mess with the O2
sensor, or even reseat the connector.... but its gone. Funny thing
is, that although there are no ignition OR fuel control ( none of
any sort) codes being set, I still get the CHK ENG lite for a brief
time at start-up as before. I do have an ABS lite that is on all the
time.... I think the sensors are dirty.....but I had that lite on
steady for a long wihile before I started gseeing the CHK ENG lite.
I have now checked the 02 connector for possible intmt
problem...but it is still like new almost, and is tight.
I guess I might as well install a new O2 sensor anyway, as it is
probably way past due.....but I would like to know what is stll
causing the CHK ENG lite on to come for a short time after starting.
Any guess?
Thanks Geronimo