what to look for when po440 evap ctrl systems malfunction code comes on on 1999 Chevrolet Tahoe

will not pass smog test.I have changed o2 sonsors in exhaust system.

Asked by for the 1999 Chevrolet Tahoe
We just use a smoke machine to find leaks in the evap system if the problem is not a solenoid. Check your gas cap first though.
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Other than checking gas cap to make sure its tight and its seal is not worn out that's all one can check without getting into evaporative test. In my opinion evaporative leaks are the most challenging to diagnose. I am not in CA. so I do not know Smog rules there...but my guess it will not pass. I suggest you take it to a professional that is equipped and has knowledge to repair your problem.