If your vehicle is showing engine continues to sputter and run after turning the key off, here are the most likely causes and the parts you'll need to fix each one.
On modern fuel-injected vehicles, injectors that fail to close completely will drip fuel into the hot combustion chamber after the ignition is turned off, causing the engine to sputter and diesel.
On older carbureted vehicles, an idle speed set too high leaves the throttle plates open. This allows the engine to continue pulling in air and fuel after the key is off, igniting off hot carbon deposits.
Installing spark plugs with a heat range that is too hot for your specific engine can cause the plug tips to glow red-hot, acting like a glow plug and igniting residual fuel after shutdown.
Excessive engine temperatures can create hot spots inside the cylinders that auto-ignite fuel without a spark, causing the engine to run on after the key is removed.
A stuck-open vapor canister purge valve can allow raw fuel vapors from the gas tank to be sucked directly into the intake manifold, providing a fuel source after the engine is turned off.
A worn ignition switch or a sticking control relay may fail to cut electrical power to the fuel pump or engine computer when the key is turned off, keeping the engine running.