Continuity mistake: When Carmen gets kicked out of Allendel in season 4, George and Angie suggest Saint Teresa's. Carmen says no because it's an all girl school, but she ends up going. In season 5, she says things like "the girls will vote for me, and some of the boys too" (when she was running for class president). She also told Angie about a boy she liked whose locker was right next to hers.
George Helps Ernie See the Cellu-Light - S5-E18
Continuity mistake: Angie is eating chocolate cake before Ernie comes over. Carmen is talking to Angie, and Angie has two big pieces of cake by her fork, but in the next shot, the two pieces are completely gone, and we don't see her put it in her mouth.
Answer: His last appearance was in the episode "God Needles George. He had a tumor in that episode, but in the end was better, and that was the last appearance of him on the show. He was occasionally mentioned afterward, so it is assumed that they still had the dog for the rest of the series.