Cool air is coming out of the vents when I put the heat on
I suspect the control valve is stuck. If your Infiniti is still under the bumper-to-bumper warranty have a dealer look at it.
This symptom could be caused by very low coolant levels. If that turns out to be true, let the dealer diagnose why, and then fill and bleed the system.
I thought of low coolant but the OP didn’t mention overheating or running hot. There may be air trapped in the heater core though.
The HVAC system in your vehicle has two separate blend doors. One for the drivers side, and one for the passenger side.
Not one common blend door.
The blend doors are what control whether hot or cold air enters the cabin.
The two things that control the blend door positions are the HVAC control module,
And the body control module,
Unfortunately, the only way to determine if either of these is the cause of the problem is with a factory level scanner.
Tester