I changed the spark plugs and the engine light stays on
Engine light stays on on 2004 Ford Focus
I am assuming the "check engine" light is the one staying on. That means the engine computer has detected a fault. There will be a fault code stored in the computer which will help lead the the cause of the problem. It is just about impossible to make the correct repair without knowing what the fault code is. Please check the repairpal encyclopedia for more information about your check engine light.
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Check the pcv hose from the intake manifold as they will collapse and leak setting the engine light on. Disconnect battery for about ten minutes to reset.