A common thing we seem to be hearing nowaday is that China has considerable control over North Korea. However, I believe this is being overrepresented. People don't also seem to know which industries China has the most control over when dealing with North Korea.
The first thing we know is that since Kim Jong-un assumed power, there has been a large push for the domestic economy of North Korea. We've mainly seen factories open up under this reign, which do actually seem to be producing things but are having trouble being sold due to the quality of them. This means that North Korea doesn't have to primarily rely on China for the production of consumer goods.
Not only is this domestic push a target for the economy, but the same has also been happening with their military. They now have the capability to produce plutonium on their own. This means that even if China halted all supply of plutonium to North Korea, it may temporarily slow down the speed of their nuclear programme, but in the long-term it'll have little to none effect. This means that in this regard, China doesn't have a lot of control over North Korea.
North Korea are relying less and less on the outside world for food as well. In Kim Jong-un's first year of power, he instituted reforms which liberalised the farming process. Since then, North Korea has been able to almost feed all of their population, even during droughts. Even if China cut off all supply of food to North Korea, it wouldn't likely result in a famine like the one in the 1990's.
Given the above, North Korea have been semi-able to produce the bare basic necessities for their economy. If there were to be more sanctions targeting consumer goods, it would ultimately falter.
This doesn't mean China doesn't have any control over their economy. They do, but it's no longer control over their missile nuclear programme or production of food and basic consumer goods.
China have control over luxury goods, and the resources that go into them. A good example to use with them enforcing sanctions is with the price of petrol in North Korea. Recently, China have been limiting the supply of petrol into North Korea. This has resulted in the price of petrol shooting up. However, targeting petrol can only go so far. It does primarily affect the elites who drive BMW's, Audi's and other luxury vehicles. They are not at risk of losing their petrol any time soon. Presumably, the regime would just direct all remaining fuel to Pyongyang for the elites to use. The local populace, on the other hand, would just go back to transporting goods using log-burning Soviet trucks.
China preventing luxury goods from entering North Korea would be more disastrous to Kim Jong-un and his inner-circle than any other possible sanctions. It affects Kim Jong-un's gift politics, which he uses to reward elites with. It also eliminates a key source of revenue to the regime, which comes from the burgeoning middle class who buy luxury goods from stores that go to funding the regime.
In fact, the regime has been getting so wealthy from this that they're constantly opening up new malls in the capital of North Korea. A prominent example of this wealth can be seen from the fact that they're opening a new golden tower with a mall which goes to fund the regime and its ambitions.
China still does have control over North Korea, but not as much control as it once did. It's impossible for them to now stop the nuclear programme using trade, but they can still do damage to the inner circle of Kim Jong-un by preventing luxury goods from entering the country. But at this point, is it really worth it anymore?