Check engine light, code 4444

[Antenor Augusto de Carvalho <antenor@ics.uci.edu>]

Hey guys,
I have a 1990 90, I-5 (NG engine), 140k miles.
I'm having a problem with the check engine light, which lights up under the following
conditions: 1) cruising, not much throttle action, engine around 2500 rpm;
2) oil temperature above 80 degrees.  I  installed a new Bosch  O2 sensor
three months ago, so I assume it's working fine. This problem began after I
replaced the old crankcase breather hose, and replaced it with the
kit that includes a metal curved tube (part # 54103227A). The old one
was falling apart but since this replacement the check engine light
is acting weird.When I pull the diagnostic codes I get 4444
(no fault, or fault not diagnosed).
What could be causing this behavior? I tested the resistances among the
knock sensor connector pins and they are infinite (supposed to the OK,
according to Bentley). The ECU ground in the intake manifold seems OK
(0.1 ohm resistance between connectors and the manifold body). I would
like to check the air temperature sensor, but could not find it. I'm running
Valvoline Durablend 20W50, oil level OK, all fluids OK, gas is 92 octane.
What really puzzles me is why do I get this check light on and
a 4444 code?
Any BTDT's?

Thanks,
Antenor