My mileage has been going down lately and have read that possible causes could be Throttle body sensor, maf sensor, or egr..if any of these or other parts that might effecting the mileage are bad..is there always a code store in front of the car. I have no codes but still am having problems with mileage. or should i be getting any codes from inside the car and take it to the dealer.
if anyone has any experience in replacing the above parts or even the vacuum lines and has some picture on how to do it ..i like to starting replacing parts one at time..i like to know which vacuum lines need to be checked..not sure how many there are and where.

thanks