1990 Caravan failed emission test NxO

My 1990 Caravan work van failed one aspect of the emission control test.
Hydrocarbons and CO tests were very good but NitrousOxide failed.
Causes, they suggest, might be lean fuel mix, EGR valve or catalytic
converter. Engine control error codes show 12 23 and 42.
How would you proceed?

 

PS It's a 3.0 FI engine here.

 

12: Start of codes
23: Air charge temperature sender improper value
42: ASD and/or fuel pump relay circuit (Z1) fault

Your "very good" HC and CO combined with high NOx suggests you're running
lean, which could indeed be caused by the two faults your computer is
squawking about.

 

Just took my ’88 Caravan for the emission test and it failed as well
for the same reason. HC and CO were fine but the NO was the reason for
the fail (1156ppm limit was 979ppm). I’m gonna pull the codes (if
there are any) later tonight. I might just fix the EGR and get it
re-tested...don’t really want to change the CAT unless I really have
to.

 

Had no error codes on the caravan. I’m just going to test and change
the EGR if it isn’t working properly according to the Haynes manual.