I have a '94 Accord EX w/ 160k miles -- recent Texas inspection failed NOx ppm on low speed dyno test also failed CO % on high speed dyno test--- no misfires noted -- car cranks- runs fine in town or highway -- does have slight valve clatter under load..
Hydrocarbons passed 93 ppm high speed -- 11 ppm low speed so not running rich or misfiring..
CO% was 5.43 high speed -- .08 !!! low speed
CO2 was 11.3 high -- 14.6 low speed
NOx ran 220 ppm high speed and 1212 ppm !!! low is this poss pre-ignition & not getting a full clean burn ?? ??

This seems to me to be more of a timing issue ?? any thoughts ?? T/U last year w/ new plugs /cap/ rotor & wires - Distributor was also replaced due to internal oil leak.