I have a 1994 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 didn’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.