int upperValue;
int lowerValue;
char ch;
int poti = 0;
int sensorValue;
Some nice global variables...
do
{
sensorValue = analogRead(poti);
int upperValue = sensorValue + 2;
int lowerValue = sensorValue - 2;
One global variable and two new local variables...
} while (sensorValue < upperValue && sensorValue > lowerValue);
These names refer to the GLOBAL variables.
OP: You REALLY need to learn about scope.