1995 Pathfinder fail emissions

[73project]

I have a 1995 Pathfinder and it recently failed emissions for high NO. The readings were as follows:

15 MPH test
HC – measured 25 with a limit of 123
CO – measured 0.0 with a limit of 0.80
NO – measured 3551 with a limit of 1367 (failed)

25 MPH test
HC – measured 25 with a limit of 115
CO – measured 0.0 with a limit of 0.73
NO – measured 2954 with a limit of 1167 (failed)

The Pathfinder had a check engine light earlier and I took it to a local mechanic that stated that the code was for a left bank lean condition. He tested the pump and it wasn’t producing the required pressure, so he changed it and the filter. The engine light illuminated again shortly after the pump had been replaced and I found one vacuum line that had a small pinhole in it, so I replaced that. The CEL hasn't come on again, so I went to the emissions place to get it tested. The results above are what came of that. I started with some basic tune up work such as replacing spark plugs, cap, and rotor. The plugs looked like they had been in there awhile, but were a nice light gray color.
I’ve done some research about the high NO levels and have inspected the EGR valve along with looking for any vacuum leaks. I removed the EGR valve and applied a vacuum signal to the diaphragm. The valve flowed air freely when I blew on the exhaust connection and applied the vacuum. This is where I’m guessing where to proceed in troubleshooting this problem! I hope that someone on this board has been down this road and can shed some light on possible areas to check next.

[73project]

Solved! The check engine light came back on and the code was for the O2 sensor circuit. I ran through the checks in the factory manual. The lights on the computer indicated that it was always trying to correct a rich condition, so it was leaning the mixture as much as it could. This is what was causing the high combustion temps and, in turn, the high NO emissions. I replaced the O2 sensor and the computer lights indicated regulation between rich and lean. The emissions test result was way below maxumum levels and the slight hesitation is gone. I picked up the sensor at Autozone for $85. Hope that this helps others in the same situation. I have 130,000 miles on the truck.