What can cause a low Lambda

[mespe]

1988 190e Key On engine off Lambda pulled from X11 3 &2 is 10.6 volts. or about 15%. starting the car and left to idle lambda is around 11 volts.

Obviously something is major wrong here, but what?

O2 sensor replaced (output voltage on orig .1 on new .46, so it was bad.)

Spark plug #4 which was cross threaded and about a 80 thousands Gap has been replaced with a properly gapped Plug

So what, besides the O2 sensor, should I check/test to get the lambda within spec for KOEO (key on engine off).

And how can I test it?