Emissions NOx fail on 1991 80q. any suggestions?

["ericaugustyn" <eric@raudi.com>]

Hello all!  

Does anyone know what could cause the NOx reading to be high while HC and CO
stay _real_ low?  Is this possibly caused by a faulty oxygen sensor or a
worn out cat?

as shown in the below readings, the NO measurement doesn't have to be
brought down much.  So is there a simple way of bringing down the number
with new plugs or an oil change?  Any suggestions?
TIA!
ericaugustyn

HC = 5.30 ppm    pass = 127.0
CO = 0.00 ppm    pass = 0.71
NO = 1088.8 ppm  pass = 910.0  <----fail!!!