Can you give us some more information?
What kind of heater is it? Wattage?
What is the input voltage from your power supply?
What were the tuned results?
Did you use M500 to save the PID tuning and put M501 in config.g to load the saved results at startup?
What does the temperature graph look like?
Is it a consistent 10c offset from the requested temperature at all set temps?
What firmware version? What Duet board?
Please share your config.g and the results of sending M122 and M98 P"config.g" in the gcode console.