Air conditioner problem on 2001 Accent

[accent]

When I turn on the air conditioner on my automatic 2001 Accent, the transmission will shift really rough on low speeds. It sounds like I am stuck on a low gear when accelerating and I hear a thunk sound when slowing down. The check engine light is on solid but it will go off if I drive without AC.

What is going on?

 

[hyundaitech]

The first step in diagnosis is probably to retrieve the engine and transmission codes.