The North Korea problem is not China's fault. It is mostly America's failure to follow through on its promises.

Some history here: China formally recognized South Korea and established diplomatic relations in early 90s. Back then, what should have happened should be the US recognizes NK as part of of reciprocal actions. That did not happen. The worst part, China apparently did not request this as part of the requirements. As the result NK became extremely diplomatically isolated. Then leader Kim Jong Il started the nuclear program in order to use as a condition for US recognition. Bill Clinton negotiated, and promised NK aid and light waters reactor in exchange for stopping the nuclear program. All was on track. Come 911. Bush not only stopped delivering on the promises, also made NK as one of 3 Axis of Evil. Nuclear program resumed.

In essence, all this nuclear shenanigan is nothing but attempt to have US directly talking with NK, and ultimately recognition.