It's not entirely surprising if your pressure cooker lid is stuck occasionally. Whether it's a classic stovetop model or a modern electric multi-cooker, every pressure cooker relies on a tight-sealing lid to do its job; and sometimes it's difficult to break that seal. There are a handful of common reasons why this happens, and an equally modest handful of ways to solve the problem.
Pressure Cooking, Temperature, and Release
Before you try to fix a problem, it's helpful to know how it occurs, so a quick review of pressure cooking seems appropriate. You probably know that water can't boil at a temperature higher than 212 degrees Fahrenheit, and that its boiling temperature actually drops with altitude. That's because air pressure drops at altitude, and pressure cookers reverse that math. They trap the steam inside a well-sealed lid, raising air pressure inside the pot and bringing the cooking temperature up as high as 250 degrees.
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That pressure can be dangerous, so modern cookers have valves to safely relieve the pressure if it becomes excessive. You can also manually release the pressure in the pot by opening a valve, a process called "quick release" in pressure-cooking recipes. If you don't do that, but instead let the pressure ease as the pot returns to room temperature, that's called a "natural release." You might experience a stuck lid either way, but the causes will be different.
Pressure Cooker Lid Is Stuck
There are several reasons your pressure cooker's lid might stick, some the direct opposites of others. The tactic you use to release it will change accordingly, so there are several that might be worth trying.
Pot still has pressure: If your pressure cooker still has pressure, the lid won't release, because that would be dangerous. Depending on the design of your pressure cooker, you can either use the quick-release valve, or lift the weight from the valve if your cooker is the older rocking type (be careful, the steam leaving the pot can cause serious burns). With a stovetop cooker, you can also move the pot to your kitchen sink and run cold water over it. This speeds the cooling process and will release the lid.
Pot has cooled: Air expands when it's hot so if your cooker has now cooled (natural release) it may have a partial vacuum inside. Working the release valve a few times might allow air in and release the seal. If not, put the pot back on the heat for just a few minutes. This should warm and expand the air again, releasing the lid.
Pot has expanded: The metal of your pressure cooker's body may have expanded more than the metal of the lid, especially if you've been cooking at high heat. Moving your cooker to the kitchen sink, and running cool water on it for a few moments, may free the lid.
"Don't know, but it's worth a try": If you've tried several other methods and they haven't worked, or if there's no clear reason why it should be stuck, you can try manually breaking the seal by giving the edges of the lid several sharp raps with a rubber mallet. This should "pop" the seal slightly with each tap, allowing enough air in to break the seal and let you open the pot. Double-check that the pot isn't pressurized (open the valve again) before you do this, lest you let out a gust of dangerously hot, pressurized steam.
Gasket is damaged: One final reason for the lid to stick, especially after several months' use, is that the silicone gasket inside the lid has become damaged. If it catches in the lid's locking mechanism, it can "gum up the works" and make the lid hard to open. The only real fix for this is to use brute force to wrench the handles apart and the lid open, so this should be your last-ditch "if all else fails" solution. As with the mallet technique, don't attempt this until you've double-checked and are sure that the pot has depressurized.
Instant Pot Lid Is Stuck
The Instant Pot, and some similar electric multi-cookers, have a few additional reasons for the lid to stuck. Sometimes, after you've used the quick-release method or after it has cooled using the natural release method, the lid's locking pin — which normally drops when pressure is released — remains stubbornly in its raised (locked) position. Tapping it gently with a chopstick, or other (non-metal) implement, will make it drop.
The lid may also be locked in place deliberately, because the cooker's cycle is not yet complete (zeroes on the timer, for example, rather than the usual end-of-cycle message). Pressing "Cancel," or its equivalent on your specific cooker, will usually fix this problem. Accumulated dirt and debris in either the rim of the instant pot or the edges of the lid itself can cause sticking. In this case, the lid moves grudgingly but will move, and the problem can be addressed with a thorough cleaning once you get the lid off.