Most temperature sensor ICs have an upper limit of around 150°C, making it challenging to find one that can measure up to 400°C. Users suggest alternatives like thermocouples, particularly "J" or "K" types, which are commonly used in industrial applications. Non-contact infrared optical temperature sensors are also recommended for high-temperature measurements. Concerns are raised about the feasibility of finding an IC for such high temperatures, as the original poster has struggled to locate one. The discussion emphasizes the importance of selecting the right type of sensor for extreme temperature applications.