The clearest sign a pressure cooker gasket needs replacing is steam escaping from the edges of the lid; with a sound gasket, steam only comes out through the valve, never around the sides. If you stretch the gasket with your hand and it doesn't spring back into shape right away, the rubber has lost its elasticity and it's time to replace it.
Steam leaks and pressure problems
With a healthy gasket, all the steam exits through the valve; a thin, steady line of steam along the edge of the lid means the seal is no longer tight. When that happens, the cooker can't hold pressure properly, and food takes longer than usual to cook, sometimes never reaching the pressure it should.
If the pressure indicator, where the cooker has one, doesn't rise at all or rises late, the gasket is the first thing worth checking; more often than not, the problem isn't the pot itself but the worn rubber part.
Checking the gasket by eye and by hand
When you remove the gasket and take a closer look, cracks, hardening, stickiness or a warped shape all point to rubber that has reached the end of its life. If it doesn't snap back to its original size soon after you stretch it gently, it has lost its elasticity; a sound gasket recovers quickly after being stretched.
A burnt-rubber smell coming off the gasket, or a shiny, tacky film on its surface, are also signs that it's time to replace it.
How often should it be replaced?
Even without visible damage, manufacturers recommend replacing the gasket every 1 to 2 years, or after around 200 uses; the rubber can harden and lose its elasticity from repeated heat cycles without any visible warning. A cooker used often may need it sooner than that.
Replacement gaskets are usually sold specific to the cooker's brand; forcing in one from a different brand can increase steam leaks instead of fixing them, since it won't seat properly.
Does a steady jet of steam or water from the valve also mean a gasket problem?
A thin, steady wisp of steam from the valve is normal, but a thick column of steam or water spurting from the valve usually points to a blocked or faulty valve rather than the gasket; the two parts should be assessed separately. If the problem continues after cleaning and retesting the valve, the valve itself needs replacing.
On some pressure cookers, the safety valve (over-pressure valve) is a separate small rubber part from the main gasket; once it hardens, the cooker may either never release pressure or release it too early. Knowing this part is sold separately from the main gasket makes it easier to order the right replacement instead of confusing the two.
A faint rubbery smell in the first few uses after fitting a new gasket is normal and fades after a few cooking sessions. If that smell lingers or turns into a burning odour, it's worth suspecting the gasket was fitted wrong or isn't the right model for the cooker.
Lightly greasing the gasket with cooking oil before fitting it helps the rubber seat into the lid easily and slightly slows how fast it dries out; this doesn't extend the gasket's life indefinitely, it just stretches the replacement interval a little. The lid shouldn't be forced shut before the gasket has settled fully into place.
Some manufacturers recommend renewing the gasket automatically every year, while others define a more flexible interval based on how often it's used; checking the cooker's manual gives the most accurate timeframe. If the manual isn't at hand, this information can usually be found on the manufacturer's website using the model number.