91 ls smog issues...HELP

[Tha Kurliest]

when it comes to smog im clueless...i just replaced the o2 sensor, set the timing, and replaced the cat no more than 3 months ago....on the emmission test my da passed the HC and CO (co measured at 0.00), and barely passed the NO at 15mph but failed at 25mph...the smog tech says it failed because its running too lean...this is the second time it has failed smog and im starting to run out of ideas...any advice would be greatly appreciated

 

[emd513]

Fuel pressure regulator. If your running a cold air it makes you slightly lean. You can get one for prox 50 bucks on eBay or local custom/performance shop. Best to get a gauge to so you can get it close enough to pass then set it where ever it runs the best

 

[Tha Kurliest]

thanks i'll try that....quick question, if i go back to the stock air box would i even have to deal with the fuel pressure?