I'm on my second ECU and IACV replacement for my 2001 Maxima. I've already had the ECU repaired and installed a "new" IACV and MAFS.

Q: What are the symptoms of needing to have the ECU reprogrammed?

1. Does nothing happen when you turn the key?

OR >> 2. Does the engine turn over, but not start?

 

I don't understand your questions. To put it better, they don't make any sense to me. A bit more history may help. So, you replaced your IACV and had your ECU repaired:
What happened after the first replacement? Did the car start/run?
What happened after the second replacement?

What happens when you turn the key? If nothing happens (not even a click), then the problem is usually in the battery/starter area. I suggest you check your fuses, the starter motor, and cables from battery/alternator to starter. Also, the neutral/clutch safety switches and the ignition switch.

If the car "cranks but does not start", then I suggest searching for many threads on the subject (in quotes). A short list of potential causes: a) NATS - you car might be immobilized (the red “security” LED is ON solid when you crank); b) Fuel Delivery - does the fuel pump run when you turn the key on? c) No spark - pull a spark plug and verify it’s firing when you crank the engine. d) Bad CPS - this sensor provides the ECU with feedback about engine speed and position. If the ECU does not its signal within a few seconds of starting, it shuts the fuel pump off. e) IACV. Does the engine start when you give it a lot of gas? If so, your IACV is bad or it failed again.