they are the ones before the cat i believe ... located in the exhaust. they arent too much fun to change but definately i would do them your self and save the money. its a simple process just hard to get to. i just but the truck on jack stands and used a 22mm stuby wrench.. the connectors are behind the eninge block.. cant see them really so just go by feel. its a good idea to use pb blaster or some kind of penetrating lube on them to loosen them up.. may want ur exhaust hot also.