3 month old puppy housebroken then not

Question
I have a 3 1/2 old male puppy. He was doing great, always defecating outside. Then we got a Doggy door to the backyard. Which he loves and uses all the time. However, since he got the door, rather than doing his poop outside, he has started doing it in the basement. Always the same two places.
  I was out with him in the back for over an hour this afternoon, he did nothing, even though I encouraged him. Then when we came in, I turned my back and he went downstairs and used the basement again.
  I am at a loss. How do I get him back on track. It is like he is holding it and waiting for my back to turn so he can use the basement.
 Help

Answer
Get him back on track by blocking his access to the basement.  Close the door or put up a baby gate.  Crate him or find ways to keep him in the same area as you at all times.  Usually dogs give pretty clear signals as they prepare for a bowel movement - sniffing, circling, hunched back.  If you see those signs, take him to the dog door or back door and go outside with him.  Praise and reward with a treat when he eliminates outside.  Leave a few of his "piles" in the yard so he can associate that area and return there for pooping.

My guess is that when he's in the basement, he doesn't have time to make it to the dog door and just goes where it's more convenient for him.

If you can go 3 weeks without an accident in the house and you clean the basement area thoroughly with an enzymatic cleaner, you can allow him access to go downstairs, but not without supervision.  At this age, you probably don't want him wandering off by himself anyway.  If he's out of sight, assume he's into trouble :-)

You may have to go back to your original housetraining protocols pre-doggie door and be sure he understands that all pottying is done outside.

Good luck and let me know if you need further clarification.