Using Nest with air handlers

This Mitsubishi unit is not compatible with Nest Learning Thermostats. the indoor and outdoor units communicate with a 2 or 3 wire communication protocol. There should be a Mitsubishi wired remote control that acts as a thermostat on your wall.

Nest thermostat is designed for non communicating systems that use industry-standard wiring that operates relays as follows:
R terminal is for 24v. AC power
C terminal is for 24v. AC common
Y terminal is connected to R on a call for cooling
W terminal is connected to R on a call for heat
G terminal is connected to R on a call for fan operation
The heat pump operation is slightly different. but uses the same principle to connect R terminal to various other terminals or operate reversing valve solenoid and relays for fan and compressor.

I'm interested in how you connected the Nest to the Air handler?!?

Turn off the Nest learning/auto scheduling & set it to hold one temperature, to see how it acts. If it can hold temperature like that, it kind of points to an interaction between the Mitsubishi control algorithm & the Nest learning function.