My car had been just squeaking pass california smog inspection, with nox readings in the 700s. Max for california was high 700s. I took the car in for inspection today, and waited, pacing, expecting to be told that ,this time ,it had failed because of high nox.
To my surprise the nox readings were in the single digits(29@15mph and 69@25mph. Wow!
When I went back over the things I'd done, since the last smog test,which would explain such a radical change I came up with this : In the process of trying to eliminate an approx 3mpg drop in my gas mileage, I found an exhaust manifold leak. I found the leak after removing the heat shield and going underneath the car with a good light,and sure enough, it was just a little black spot where the manifold and head came together. Something you'd really have to look for. I didnt expect that repair would have such a dramatic affect on nox readings.

All comments and observations are welcomed.

Very nice! That's valuable for everyone to know.

What else did you do between smogs? Anything with an EGR (if you have one)?

Happy Bricking, especially with your mileage increase ;)
William
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1990 740 Turbo, on its way to stock specs, maybe beyond

I put in rebuilt Amm. No noticeable mpg gain with that. Then just regular maintenace. I do have the egr but I didnt touch it. Except for removing the pipe from the manifold, and putting it back when I resealed the manifold. EGR Pipe appeared properly attached as I removed it. Not leaking.
All is tight now, and the car runs much better.
To find those leaks you have to get down there and do some serious eyeballing. Lots of little cracks and crannys.
A small inspection mirror and a good source of light as well.

Happy motoring.