I have a 91 Hf with a 89 D16A6 that failed emissions. It passed at 15mph but failed at 25mph due to high NO. (Mods: AEM Intake, 1 Piece DC D series Header)
Timing: 18 BTDC
O2 sensor - a few months old
Cat - Half a year old

Comprehensive Visual Inspection = Pass
Functional Check= Pass
Emission Test= Fail

Emissions numbers:

15mph
HC = 66 (max=134) pass
CO = .31 (max=.82) pass
NO = 833 (max=847) pass

25mph (Fail)
HC= 17 (max=108) pass
CO= .01 (max=.71) pass
NO= 925 (max=786) failed


Any Suggestions? What might be the cause of high NO?