1997 Max SE with auto climate control, AC had been working fine now it's suddenly blowing hot air. When I turn on AC, compressor kicks on, belt is intact, both rad fans turn on, Low pressure line is ice cold. What do I check next?

 

You probably have a leak in your system. ID add refrigerant. But ID check your refrigerant pressure first to make sure you have refrigerant

 

Get down in the passenger side footwell. Remove the plastic cover that covers the end of the ECU. Turn on the ignition key (you don't have to start the car if you don't want to). Watch the little arm right over the ECU as you change the temperature setting on the heater control panel back and forth from 65ºF to 85ºF.

If the arm doesn't move, the motor that moves it is bad. It is called the Air Mix Door Motor.

If the arm moves, run the self diagnostic and see what the results are. To run the diagnostic, follow the directions that start on page 70.

diagnostic - http://boredmder.com/FSMs/Nissan/Maxima/1997/HA.pdf

 

R134 might be low.

 

check refrigerant level

 

If the low pressure side is cold, the system is working fine. Still, verify you have correct line pressure.

If pressures are good, then you air mix door is fubar like Dennis said. Check operation of the door. U can usually hear the door open and close when u cycle air temp from cold to hot.