Just wanted to introduce myself to the group. I just purchased a used 2015 Frontier with 27000 miles on the clock. I love the truck with one exception....it idles like a tank, especially with the AC on.
Once it's warmed up base idle is around 630 rpm. In drive the idle is rough and is worse with the AC on. There is a rattle under the dash on the passenger side at idle. If I barely press the gas and raise idle to about 750, no more issue. Been to Nissan 5X in the 6 weeks I have had the truck. I asked them....set base idle at 700 or 750 and I will get out of your hair. They just will not do that.
Well, I am certainly not an expert on these trucks (by a longshot). First, the service manual I have (for a 2013 Frontier) shows 700 rpm base idle +/- 50 rpm for the automatic in N or P. That means the range of acceptable base idle speeds is 650-750 rpm. It would make sense (logical) that the AC would make the problem worse, because of the additional load on the engine while it is already idling too low (630 rpm) without the AC on. I'd take the path of least resistance and find an independent repair facility specializing in Nissan and ask them to turn the idle up to 700-750 rpm. Do you have the service manual for your truck? It is available online...Also, I have an OBD reader permanently installed with an app on the wife's iPad...might help you in diagnosing and is very inexpensive (less than $30 reader and app).