93 D21 FAILS SMOG HIGH NOX

[oleman]

My recently overhauled KA24E is failing Texas smog test (same as federal and mandated by the epa for this area).
The engine was bored .020 over and the head and block deck were cleaned up. So the compression was raised but the engine runs fine on the same low octane fuel it had been using.
I do not recall whether the cam was replaced but if it was it was specified to be stock profile.
I made an effort to keep all smog items intact during reassembly and replaced all parts that looked worn.
The CAT was replaced in 2010 so I kept it.
I have the timing advanced about 2-4 degrees
It passed the smog test in 2011 about 6 months before I overhauled it.
This the first smog test since total engine overhaul.

This is the second failure of the smog test.
The first test test failed with alost the same results.
I brought it back home and discover the lime from the egr amp to the egr was off.
I put on an new line, thinking this had to be the problem, took it back of the retest.
Very suprised to see the essentially same results.
Actually the NOX went up a small amount.

(15 MPG)
HC STD 191 CURRENT 34 PASS
CO STD 1.63 CURRENT .43 PASS
NOX STD 1323 CURRENT 2662 FAIL
DILUTION STD >6 CURRENT 15.5

(25 MPH)
HC STD 186 CURRENT 10 PASS
CO STD 2.11 CURRENT .56 PASS
NOX STD 1208 CURRENT 1940 FAIL
DILUTION STD >6 CURRENT 15.8

Any good guesses to what is causing a test failure on the 93 D21?

[oleman]

I did the plunger test on the egr valve it appears fine.

[Motormouth559]

A slow or bad o2 sensor will cause high nox numbers