Ignition Key OFF but engine still running?

First of all confirm this issues.
Has someone hot wired your fan ?
To be on with key on? does it stop on its own (say after 3-5seconds after the key is turned off) or does it just keep on running?
OR
Can you shut it off in gear and does it stop then?

Check this possibilities:

If the engine has not been tuned in awhile,the start with a good throttle body cleaning and a plug change, add some fuel injector cleaner and give it a day to work in.Also in some cases its seen that  carbon bulidup on the tops of the pistons / quench area of the head-
Also see that does your idle seem high?If yes then inspect Idle Air control Valve.

If you see check engine light is steady ON dash.
Then your car needs scanning.
Getting the engine computer scanned will help to confirm the problem.

This will help.
Thanks.

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