Our front yard was full of weeds. We cut the grass & put down weed & feed & started to water it. We now have yellow patches. Does this mean it's working on killing the weeds or have we done some other damage to it?

Lawn weeds can be very competitive against preferred grasses, depending on the time of the year. Typical weed and feed products act upon either broadleaved or grassy weeds in the lawn. The yellowing patches could be an indication that the herbicide component of the weed and feed is working. Take a close look a the yellow patches and if all the same kind of plants are yellowing, the chance is good that it is working. it should be noted that some grasses may yellow while the weed control products are active and return to a green color later. To get a more local opinion on what is going on, I suggest you contact your local county extension service.