How to I get my dog to quit peeing in the house?

I have a 5 month old chihuahua. He is pee pad trained. We've had problems with him peeing in our bathroom for a few months, and just recently in our computer room. Our vet said to just keep the doors shut and keep and eye on him. We did that for about 2 months. I thought he was trained because he only went on his pad. We recently opened the doors back up and he immediately started peeing and pooping in those rooms again. We've cleaned the areas thoroughly, but it doesn't seem to stop him. Why does he do this, and will he ever stop?

keep it outside

My friend kind of has the same problem so he kept yelling at it and bringing the dog outside so it can pee there and eventually it worked

We have a mini dachshund and she does the same thing. We were told by our vet that if she starts peeing anyone but on her pad to pick her up IMMEDIATELY even if in mid-pee and bring her to the mat. We usually say NO and she'll stop. We take her to the mat and that works. Good luck! Getting your dog to stop peeing and pooping on everything you own is quite the challenge.

take it out right after it eats or drinks and every couple of hours, puppies have small bladders and usually go soon after eating and drinking