S it hard to change an oxygen sensor in a 2002 mercury sable?

ok so my boyfriends check engine light came on he took it to a place to get checked out and one place said it was the maf sensor so i thought no biggie pretty simple but then another place said it was the oxygen sensor which kinda threw me off since i was new to the maf sensor so i researched figured out how they tie into each other but how can i find out which it is and if it is the oxygen sensor is that hard to change out?|||Sounds like a lean code going on, Look for a vacuum leak, or the MAF is dirty and can be cleaned with brake cleaner by spraying a little on the 2 thin wires in it, being careful not to brake them|||It is easy to change,just a 7/8%26quot;or 22mm wrench,but have to know which one,there are four of those,have to know the trouble code number (s)to see.