My wife's 04 Maxima transmission went into what the dealer called "Safe Mode", where it stuck in 3rd gear, and couldn't shift up or down.
I found this site, and was concerned the dealer would jerk me around, like it appears they've done so many of you. So I had a family friend, who owns a transmission shop, run the diagnostic. He was familiar with the Nissan transmission, including its issues, but found the problem to be 1 of 3 "positioning sensors". The diagnostic won't tell you which sensor failed, so you need to replace all 3 to be safe.
Now here's the good news, unless you take it to the dealer, the sensors cost less than $60.00, and I was back on the road for under $300.