Owners of Nissan Sentra and other models have complained of air-conditioning problems.  These include blowing hot air. Attempted repairs have included replacement of the heater and air-conditioning control, air-conditioning assembly, and other components.   Other causes may include leaks or compressor issues.  One site suggests, this checklist  for air-conditioning issues,

Using a UV A/C leak detection kit is the easiest way to find a leak.
Follow the instructions on the packaging to introduce UV dye to the system and UV light to find the leak.
Check around all fittings to verify that they are all secure.
Check hose manifolds on the compressor.
Check the front seal and the o-rings sealing the pressure switches on the back of some compressors.
Check where the hoses are crimped onto the fittings.
Check the Schrader Valves.
Check for pin holes in the condenser.
Check where the evaporator drains condensation with UV light. Sometimes oil or dye can be seen.

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