This is a continuation of a post with the same name. Briefly, the 91
Hyundai Excel I own idles roughly and smokes sometimes even though it
seems mechanically OK (cylinder pressure within spec).

I am hoping COMBOVERFISH responds to this post as he was absolutly
right in suggesting a multimeter would show a trouble code when a scope
did not. After the last post, I replaced the oxygen sensor and the
symptoms are similar.

Diagnostic Codes
1 2 5 2 5 2 5 2 5 ... which I take to mean Barometric pressure sensor.
So far I have no idea where this is or how to test it.

Oxygen Sensor
I replaced the sensor although a good ground may have prevented me from
detecting that the old one was working. In any case the behavior is
unchanged. On warm start, I measure between 150-300 Mv initially. After
5 minutes the signal has increased to the range 890-1000 Mv. The engine
idles roughly and when accelerated, smokes. Unhooking the oxygen sensor
seems to produce no change in the smoking behavior or the oxygen sensor
reading.

Am I reading the codes correctly and what is the barometric pressure
sensor?
Is it possible that the oxygen sensor is not connected to the ECM?
Any suggestions happily accepted.
Thanks