Hi Everyone,
You guys were so helpful last time, I had to come back. 
I’d appreciate any thoughts you have on this…
About a month ago, the SES light came on in my 99 Nissan Altima GLE. Autozone (“AZ”) connected its OBDII diagnostic machine to my car, and it produced a P0400 code, which is related to the EGR valve sensor. AZ guy said it could mean any of four or five things. He advised I first replace the EGR valve gasket, since it’s the cheapest of the possibilities, and in his experience has often been the cause of that code. I replaced the gasket.
The problems now are (1) how to reset the SES light, and (2) if it comes back on, how to most inexpensively identify the real problem.
On (1), must the SES light in fact be “reset,” or would it turn off on its own if the real problem were fixed? If I need to reset it, how do I do that? I see nothing in my manual about this. I did find instructions online for how to reset the SES light on close models (not exactly the 99 Altima, but other Nissans around the same year). I tried one set of instructions, but it did not work (turn the ignition key to a certain position, press the gas pedal X number of times, etc.). Someone said online that AZ will reset it with the same OBDII machine. I called AZ, but they said they don’t do that (sounded like a matter of company policy). A friend suggests I might need to pay a dealership to reset it, but that sounds a little strange–would the third/fourth/fifth owner of a car over its lifetime really need to seek out a dealership everytime to reset this light when it came on in order to see if whatever he did really fixed the problem? The friend says I might need to have the dealership run its more specific diagnotic computer on my car anyway to find out the real cause of the SES light coming on.
On (2), if I knew the light were truly reset yet “came back on” (whenever that would happen), does it sound like I should…
-just go ahead and pay the dealership $80 or so to run the diagnostic to find the real problem?
-try to address another inexpensive possible cause of the P0400 code?
-trust that the EGR valve needs to be replaced? (though I pulled up on the flow sensor inside the valve and the car began to stall, as I understand it should do this if it is functioning properly)
-not do anything until I detect an operating problem?
The car seems to be operating fine. Engine temperature is normal. I noticed no change in behavior since the light came on. I’m not sure if I’m imagining this, but the sound of the exhaust system might have gotten slightly louder since this all happened.
Thanks a bunch for any advice you can offer.