Nissan - Maxima :: 2000 - Service Engine Soon Light

I just purchased a 2000 Nissan Maxima GXE with 219,000 miles. I've had it less than two weeks, and the Service Engine Soon light just came on. I live somewhere that requires emissions testing in order to register the vehicle. My temporary plate will expire soon, so I need to get the car to pass emissions.

This is my first Japanese car and first non-American vehicle. I didn't know until after I had given the check to the guy selling the car that he used premium gas. He said that Nissan recommends it. I've never driven a car that required premium gas before and wasn't keen on the idea since here, it is 40¢ a gallon more expensive than regular unleaded.

I spoke with my landlord about it, as he is a certified Honda mechanic. He told me to first try getting half a tank of regular unleaded and putting it in with some fuel injector cleaner. He said if I didn't notice any decrease in performance or knocks, to then just add a bottle of of fuel injector cleaner to every other tank of gas.

I got a half tank of regular unleaded with a bottle of fuel injector cleaner and didn't have any issues. I then filled the tank with regular without any fuel injector cleaner. I used about ¾ of that tank when the service engine soon light came on. I took the car to Valvoline to get the codes read, and while they couldn't give me any code numbers, they said it was something to do with emissions.

Here are my questions:

Is it possible the light came on because I switched from premium to regular unleaded or perhaps because I would need to add fuel injector cleaner to every tank of gas I get and not every other tank as my landlord said he does with his car?

Should I try either adding fuel injector cleaner to every tank of gas or just switch back to premium? If I try either of those, and one of those was the problem, how long would it take for the Service Engine Soon light to turn off?