No, it's not necessary to have a candy thermometer to make this caramel sauce. You can make a great sauce just by paying close attention to the color and smell. However, if you have a thermometer and want to be very precise, aim for a temperature between 338°F (170°C) and 350°F (177°C). If the temperature goes above 350°F (177°C), the sauce may start to smell like it's burning and could taste bitter. If it's below 338°F (170°C), your sauce may not have the proper color or sweetness.