inputReading1 = analogRead(sensor1); //getting the voltage reading from the 02 sensor
So, inputReading1 could be 0 or 1023 or anywhere in between.
Input = analogRead(PPO); // PID analog input for PPO
Gets changed, by the precompiler, to:
Input = analogRead(inputReading1*aref_voltage/1024); // PID analog input for PPO
Since aref_voltage is 1.72, you will be reading from pin 0.0 to pin 1.718. Does that make sense?