While the MAX31865 can support either 100ohm or 1kohm RTDs, the Adafruit breakout comes in two versions with different reference resistors specific to each type of RTD.

• PID 3328 = 430Ω for use with 100ohm RTDs.
• PID 3648 = 4.3kΩ for use with 1kohm RTDs.

If you are unsure which one you have, you can check the value of the reference resistor by reading the label. See below for details.

Also, be sure to change the value for RREF in your code to match.