Code 12? 1995 240sx

[RS91]

My check engine has been on since I got the car last month. I followed the article about reading the code on DOHC engines. Without starting the car on Mode 1 I got code 12 which is Mass Air Flow sensor. I did not bother to start the car to check for more codes. Do I have to start the car to get the correct code? What is the difference starting and not starting the car? The check engine is now off, should I replace the MAF or should I wait for the check engine to come on again just to make sure is not a glitch on the ECU?

Thanks