Check engine light was on so it didn't pass smog

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I took my 2001Toyota Tacoma truck in for a smog test .. I had no worries it has always passed ..after five min . The tex said my truck had failed ..I asked what Was the prob. OMG a foreign lang.was all I was hearing! Until finally the only reason it didn't pass was because the "CHECK ENGINE LIGHT " was on , emissions passed and the other one sorry can't remember .. So I had to go get extension from dmv @ $ 5o. Anything more then a day extension cost $ I have to pay$ 90..for diagnostics eat to tell why light is on etc ... Any suggestions