This section?
Yes
Have you placed some Serial.prints to prove the value of ST is correct and is changing as it should?
Yes
Are 68° and 70° correct?
Yes
Is LOW fan OFF and HIGH fan ON?
Low is fan ON and HIGH is fan off........................
This section?
Yes
Have you placed some Serial.prints to prove the value of ST is correct and is changing as it should?
Yes
Are 68° and 70° correct?
Yes
Is LOW fan OFF and HIGH fan ON?
Low is fan ON and HIGH is fan off........................