If your house air conditioner is emitting a sweet smell when it's reaching the set temperature, it could be attributed to a few possible causes:
Dust and debris: Over time, dust, debris, and other particles can accumulate within the air conditioning system. When the AC reaches the set temperature, it may trigger a cycle called the "cooling-off cycle" or "fan-only mode," during which the fan continues to operate while the compressor is temporarily turned off. This cycle allows the AC to circulate air and remove any remaining moisture from the evaporator coil. If there is accumulated dust or debris on the evaporator coil, the heat from the fan can cause the particles to burn, resulting in a sweet smell.
Mold or microbial growth: The evaporator coil and condensate pan of the air conditioner can sometimes become breeding grounds for mold or microbial growth. When the AC reaches the set temperature and enters the cooling-off cycle, the moisture present on the coil or in the pan can cause the mold or microbes to release a sweet odor.
Coolant leak: Although less likely, a refrigerant leak could cause a sweet smell when the AC reaches the set temperature. Refrigerants often have a distinct sweet scent, and if there is a leak in the system, it may become more noticeable when the AC operates at its peak performance.
If the technician you called to repair the AC didn't find any issues, it's possible that the problem is intermittent or subtle, making it difficult to diagnose during a brief inspection. If the odor persists or becomes more pronounced, it would be advisable to schedule another appointment with the technician or seek a second opinion from a different HVAC professional. They may perform a more thorough examination to identify any underlying issues and recommend appropriate solutions to resolve the problem.