she won't pass smog due to high emission and engine light on, with the under the

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dash board computer says #4, but other and mostly all mechanics won't tell what it is or mean without checking it themselves....no good mechanics any more...pleade help...never had problems with her not passing, til I got my fuel injectors put in last year...nor engine light problems...always take care of her so she'll take care of me..turned around ask mechanic why light is on, he checked and i saw the #4, asked what it meant, said he didn't know... I know one thing it has to do with my negative to my battery...cause if my battery shift by hitting a bump or ruff road, the light pops on,if i move the cable it'll go off, so I have something there to keep from moving, and yes I know the cable is loose, like I said No good mechanics...again please help..THANKS