Chip Temperature

Teensy 3.x:

The internal temperature can be read out at different resolutions. Try changing the resolution and you will get very different results. Also, the readout values have to be treated with a calibration function using an intercept and a slope. These have to be empirically derived using a reference thermometer. It seems the intercept and slope values differ largely between individual Teensys.

void setup() {
  while (!Serial);
  analogReadResolution(16);
}

void loop() {
  float temperature;
  // for 10bit resolution
  //temperature = -1.8626 * analogRead(70) + 434.5;
  // for 16bit resolution
  temperature = -0.0293 * analogRead(70) + 440.5;
  Serial.print(temperature);
  Serial.println("°C");
  delay(500);
}

https://forum.pjrc.com/threads/49558-Reading-Teensy-internal-temperature-sensor?highlight=internal+temperature

Teensy 4:

The Teensy 4 core has an inbuilt function:

extern float tempmonGetTemp(void);

void setup() {
  while (!Serial);
}

void loop() {
  Serial.print( tempmonGetTemp() );
  Serial.println("°C");
  delay(500);
}

Note: Temperature conversion formula from degree Celsius to Fahrenheit is given by

F = (C * 9.0f / 5.0f) + 32.0f